Different Jupiter 8 m39 lenses

Hello!
I have two questions, sorry if the thread is in the wrong section. I have a Jupiter 8 lens but I have seen other Jupiter 8 lenses with a nob on the ring agenst the camera house. What do it do?

And my second question is what is the easiest way to check if my lens focus correctly without wasting film?

Thanks

1. It's an infinity lock, useful on screwmount lenses.
2. Groundglass at the filmplane, observe with a loupe.

Focusing tab, I guess. I don't know J8 with something else
I use ground glass, release cable to hold shutter open and macro lens, camera or magnifying loupe to check it on target in 1m. Here is also method how to check it on infinity with camera and tele lens. You should be able to google it, I can't do it on the phone.