I just obtained this Hamilton 4992B and discovered what I initially thought was an errant hair in the movement, but upon closer examination discovered it is a piece of metal, securely attached. It looks like spring material.
Any ideas on what this is? Is something happening to the mainspring, or is this spring related to the “hack” mechanism? Should I be concerned? The watch seems to wind, set and run fine.
It is located just above the serial number, transecting the “C49”.
Thanks.

The spring is part of the hack feature...When you pull the stem up to set the time this spring goes up against the balance wheel to stop the wheel..

Ah yes, thank you. Indeed, in searching pics of other 4992B’s, I have seen this “finger spring” in various positions.
I find it amazing that the watch designers would have resorted to pressing a spring against the balance wheel to stop it, the balance of the wheel being so delicate and critical (although I can’t think of another way to stop it). I would have worried about upsetting the balance (poise?) or putting undue force on the balance wheel arbor and jewels. The spring must apply a very delicate touch.