What is the purpose of a braze-on on the underside of seat stay?
I'm not positive, but I believe those are for a frame-mounted wheel lock. These are locks that you mount to the frame in roughly that location that immobilize the rear wheel when locked. Typically they mount via the brake braze-ons or their own straps, but that location looks about right for a direct mount

An example of a fitted lock can be seen in this image:



This type of lock is convenient, but not especially secure, since a thief can always just pick up the bike and carry it off. Some do include a way to plug in a cable or chain in order to lock the bike up to something else, but those aren't especially secure, either.