@Tony C. some more info from the seller, re: the case/hallmark. Curious to know what you (and others) make of this. Again, this is the cal 89 with the angled (not "sharkfin") lugs, for sale by a forum member here who's been communicating (very graciously) via PM. "according to a quick research I did the hallmarks are swiss. It is Helvetia standing for 18k gold and the letter C indicating the case was made in la chaux-de-fonds." (See attached pic.) And he shared this helpful link: https://www.vintagewatchstraps.com/swisshallmarks.php I know the other lug-style is the more collectible/desirable, but would this assuage your concerns about the case? And again I'll say thank you to all who have weighed in---very much out of my wheelhouse (...out of my watchbox, I guess I should say) here.
I was referring to this: Perhaps it's not a hallmark, but it seems a bit odd to have been naturally produced, and is certainly not typical. I would personally hold out for a photos of the inside, but if you trust the seller, and like the watch enough, you could ask for a guarantee that it will have correct markings, like this one:
I see—yeah that is a peculiar little marking. Out of curiosity, how is the caseback removed on these? I don’t think I see treads, so I assume it’s not a screw-down; is it just a snap-off? Is a specialized tool necessary? Is it the sort of thing a layperson could reasonably be expected to be able to do? In any event, it’ll be up to the seller (of course) to remove the back or not. You’re giving me enough pause here to potentially consider not doing so a dealbreaker, as I wouldn’t want to try to negotiate an international return with a person-to-person sale if I were to open it myself upon receipt and find it to be incorrect.
Yes, it's a "snap back", and they can be tricky. There are special knives that are typically used, but especially on valuable watches, it's best to have a watchmaker do the job.