Anti doubling recoil operated counter weight.
If working right it blocks the trigger upon firing in the full rear position. The shooter than must make a full release of the trigger before the left bbls selector system in this SNS system will engage.
Standard item in a single trigger set up on a two bbl gun.
A very light weight spring keeps the weight to the rear position,,then the guns recoil snaps it forward rotating it under the trigger to block it and do it's job.
If that spring is too heavy, it may not work at all (or if the load is too light). If the spring is too weak it may not reset properly and block the trigger pull itself.
The mechanism look like it works off the right hammer to set itself when the action is opened and move when fired.
Ithaca is similar
