This is an accessory for a leg vise to prevent the chop racking. There are other ways to do this - a deck of playing cards is easier - but I wanted to build it in, and not have to look for an accessory. Your bench may offer the same opportunity.
This is the anti-racking at work ...
https://i.postimg.cc/CKBphMRr/3.jpg
https://i.postimg.cc/0yNq40Ps/2.jpg
How is this done? Alongside the leg is a "depth stop" built from a screw. It can be adjusted for the desired length ...
https://i.postimg.cc/ZR6kNfFr/5.jpg
https://i.postimg.cc/BQp0NVWp/1.jpg
I designed and built this a few years ago. Rob Lee came to visit, and commented that the design needed extra reinforcement to prevent being bent back. He was correct, and I have now used angle sections to attach it to the leg. Solid as a rock ...
https://i.postimg.cc/k503Br8J/6.jpg
Regards from Perth
Derek