That's better. Back at my desk. The WOK auction on Saturday, BTW, was quite good and a real eye opener for anyone who has been at this for a while.
Tony, sorry to get under your skin on what is both an obscure point and one that is not what I came here to get help with (I posted here BTW because all of the in-depth S.58 posts were on this site). As for my original question, I am still searching. My next step is to have the odd insert and one of my standard inserts analyzed and photographed by a friend at 77 Massachusetts Ave. who has helped with these kinds of things before. Spectroscopy can be very helpful. I'll report the results. Certainly the original insert is not useful for diving as is and so an "upgrade" would have made sense for many potential reasons, some of which I've mentioned already.
As for the question that has captured your attention -- the origin of the S.58 nomenclature -- that was never my focus and, as I have REPEATEDLY said, a topic for conjecture not conclusions at this point.
To sum up our respective views:
You have your letter from the successor owner of Zenith which states that the "S" relates to waterproofing and the "58" to the year of manufacture. It does not reference any particular occurrence in 1958 that the watch is commemorating, including the geophysical year, and there is NOTHING in the Zenith advertising of the time to suggest a connection between the watch and any particular event. In spite of that lack of any express connection to anything in 1958, Zenith (per your theory) put the date on both the front and back of the watch. In fact, it is engraved in HUGE letters on the back of the watch. I am not aware of any watch company touting a random year -- or even a year of introduction (except in the anniversary context) -- in that way. It certainly would not do anything to sell watches and would look strange, indeed, on watches produced in subsequent years (the S.58 was produced for a number of years), making the watch seem oddly dated (at a time when that was decidedly not a good thing from a marketing perspective). In short, this is not a sensible theory. The only thing to recommend it is your one sentence letter from a fellow who likely has no knowledge whatsoever of the company's history.
Now, as for my conjecture, I think it has a number of interesting points in its favor. Needless to say, if the S.58 were a Timex or a Vacheron, I would not make this suggestion. However, Zenith has a uniquely deep and important history as a manufacturer of pilot watches and aviation instruments. This is a legacy that they have cultivated and touted in their advertising to this day. If you visit their website (which I can't hot link here) you will see that they have a whole section on their historical aviation instrument business. They also heavily tout their pilot watches and have just reissued the AMI chrono worn, inter alia, by Sikorsky S58 pilots. I have also received now two (totally unsurprising) reports that Zenith instruments were used in Sikorsky helicopters of the 1940s. The S.58 was NOT released as a dive watch (as some forum members here have mistakenly suggested) -- the bezel came in on later versions of the watch. It was initially a manual wind, time only watch in the tradition of the Mark XI/Explorer/Commando/Railmaster. In short, it was the perfect PX watch for a military man with the military aviation flavor provided by the unmistakable S-58 logo as an appealing style point. As for whether the very odd S in S.58 is the Sikorsky S or not, forum members can decide. To my eye it is fairly unmistakable. However, even apart from the font, any airman or marine of the time would get the message just as surely as if Zenith produced an "F-16" pilot watch today. In short, the combination of the nomenclature, the distinctive Sikorsky logo, the pre-existing association of Zenith with Sikorsky (and, more important, with pilot watches and avionics generally), and the fact that Zenith had always targeted the military market (they sold as many DH-style watches as DH watches in WW2) all add up to a very powerful circumstantial case that makes a lot more sense than anything in Marc's letter.
I think you also don't have the full picture of the business scene for watch companies like Zenith in the 50s and 60s. The withering military market was a huge issue and the military market salesman were doing everything they could to stoke sales. It was a huge sector, accounting for more than 15% of overall Swiss watch sales at the time. It also tended to be a highly profitable sector which explains why so many big companies pursued military contracts. And it wasn't just issue watches (I have NEVER suggested that the S.58 was an issue watch). It was PX watches, too. Jenny and Glycine based practically their whole business model during the 60s on base and military mail order purchases. As I have said above, it was military contracts that saved Heuer and BP would have folded a lot sooner without them. You can't even begin to understand how important the military sector was in the 1950s.
As for why Zenith never touted the significance of "S.58" in its advertising, that small problem plagues your theory as much mine. However, I would suggest that it is more of a problem for you. There is NO reason in the world that Zenith would not have mentioned the geophysical year if that was its intention. However, if they wanted to get some traction out of a famous military aircraft association without paying Sikorsky a dime, they would have been smart to keep mum. As you point out, the logo is very slightly different. I think Zenith would have lost a trademark suit, but maybe the small difference was enough to keep Sikorsky at bay (or maybe they just didn't care). But a proper Sikorsky ad campaign would have cost Zenith money.
Anyway, this is all just conjecture, as I keep saying, for now. But your theory seems unconvincing to me, while mine, although it needs shoring up with hard evidence that I will surely look for (I've been at this case for one day) seems very plausible and intriguing in the context of a company with Zenith's aviation history, its repeated reliance on allusions to that history in its marketing, and its specific connection to Sikorsky and S58 pilots. I will be calling Sikorsky in a few hours to see if I can locate someone who know a little bit about early instrument arrays and, possibly, watches.... I hope to visit them in the next few weeks. If you are in the NYC area, perhaps you could join me. I'll buy lunch!
Thanks for all of the input from folks here. If you google around other sites, you will see that some well known military watch experts see significant initial merit in my embryo conjecture. Remember, this is not so much about a specific link to the S58 as it is about marketing watches generally. And, again, the S58 did not begin life as a dive watch. Which brings me back to the question I came here to ask: I'll let you know whether the material used in my insert is identical to the material of the standard insert or not. That seems like a good starting point for that inquiry.
Best, Hurley
PS. Here are a couple of fun and instructive links per the above:
http://watchesbysjx.com/2015/04/zeniths-history-in-pilots-watches-explained-in-five-timepieces.html
https://www.helis.com/database/modelorg/730/
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