Archer
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Despite reading somewhere that the case back was a different size, I can confirm to the contrary and that a normal Speedmaster case back will fit the FOIS. I've just swapped over a spare 3594.50 case back with the FOIS and its a perfect fit, no issues. I would therefore assume that fitting a sapphire case back should be fine too.
You would also need the o-ring 088NS0607 which goes between the spacer and the sapphire case back.
If I have the time at the weekend I can probably try a sapphire case back and spacer to be 100% certain of the fit on the FOIS.

I'm on his mailing list and this FOIS was for sale as of 3 days ago, I was seriously thinking of contacting him with a view to purchase and then selling my FOIS but for some stupid reason decided not to 馃榿
My FOIS already has a steel bezel but I guess one of the best upgrades is the hesalite crystal, seen here installed by Lewis Watch Co
I think the confusion here is that the Thomas ones are aftermarket and come with the ring, it sounds like teojjsg is talking about the OEM sapphire back, presumaly from the sapphire sandwich which clearly doesn't come with a spacer ring. I personally don't much care for the rounded shoulder profile on Thomas's backs, though they do allow you to show off the movement on older models which didn't originally have the option.
?? You did it on a 321? How did you solve the rattling issue? You need a spacer for that, right? As far as I know the spacers from the 1861 don't fit on a 321. I tried to do this once and I kind of gave up. I don't want to do it on my speedy anymore, but I'd still like to know how it was done.
I never got around to doing it because I changed my mind midway through my research. However I came to the conclusion that you need some sort of o-ring or a spacer on top of the 321 movement ring and then screw in the OEM display back. I even got the measurements for the spacer one is supposed to fit in, but never got around to having the part made for me.