Hi.
In the picture here below there's a jewel in its gold setting taken from a 992B movement. It is a nice detail, chatons give a fine looking to the movement, with Geneva Cotes it's even better. I have read many times that gold jewel setting is just for making an eye appealing movement, and I accepted (more or less) this idea. But ... see next post

On the concealed side of an Omega, the side of the base plate under the dial ... there are chatons. Now if chatons are technically useless, why put them where no buyer's eye sees them? The same holds for a Tissot, and I think for many others if you simply take dials away. The picture below comes ... from a Waltham, a poor Crescent Garden built between 4/1/1877 and 5/31/1877.

Press-Fit wasn't that popular 150 years ago I think. And they're not all gold chatons, a lot of them were just brass. That underside of the plate you show looks like a "Cap Jewel", so it's probably best that it's held there. It's in a chaton because you wouldn't want to screw down directly on the jewel (too easy to shatter) to hold it there. Nowadays it might still be a chaton or some type of shock-proofing with a spring holding it.

Just going from a materials standpoint the jewel assortments I see from about 70 or 80 year ago have almost all jewels with rounded tops that are designed to be put into a chaton. The jewel assortments I see starting from around 50 years ago have the flat top jewels that are made to be press-fit. But that's just what I've seen, perhaps that's not quite representative.

I could hazard some guesses about why things changed on the uncapped jewels but that's all they would be, just guesses. Perhaps someone else has something more enlightening to say.