A relic of the Jewel Wars, this 35mm 1950s Mount Royal with an 80 jewels (kind of) Felsa 4002, 21 of which are broadly functional and 59 which, er, aren’t. I’ve no idea how many of these watches survive today or, indeed, were ever made but it “coulda been a contender” until the likes of Waltham, Orient, and Titoni climbed into the ring with their 100 (and more) jewelled efforts.
Of the two recorded makers of a Mount Royal brand, Choisi seems to be the most likely candidate for today’s curiosity, it being a product of the original company founded in 1929 before its demise, date unknown. The company name was seemingly reactivated in 2013 by a Singapore-owned enterprise which, from what I can see, is largely producing retro-style watches along with homages based on the old company’s vintage models.
Really! That means pretty thick watches, considering the additional thickness compared to a stainless steel back. Below link is a chart I've prepared from industry sources for hardness glass. For sapphire one should add about 15%. Sapphire is also a poor choice for diver watches of some credibility, because its inferior resistance to shatter. And still about credibility may be worth to mention that the best case back is the one that is'n there at all, e.g Seiko Marine Master, and many others.