Lock bar stabilizer, a round disc that prevents the frame lock from springing open. Created by Rick Hinderer
edit to add:
the Hinderer LBS actually has two functions
1. overtravel stop as mentioned
2. stabilizes the lock bar so that it doesn't flex up towards the spine during hard use.
Sometimes you'll see a knife that has two overtravel stops, such as the ZT0562.
There's an overtravel stop built into the steel insert on the lock interface, and also an LBS disk.
In that situation, the disk is functioning primarily to prevent the lock from flexing towards the spine (2nd function)