It's a remontoir d'egalite. Basically a mechanism that doles out the power to the escapement from the mainspring so that the force delivered remains close to constant, thus improving rate-keeping.Lovely FPJ. What is that little widget down the bottom of the dial?
Dino, thanks for the intel on the CPCP trimmings. The ones I was looking at didn't have all of that - not a deal-breaker, but I'm in no hurry so would be nice to wait and see if I can get a full set.