>>2928610
I've never seen that before in any Singer machine, but searching for it over the internet it should be for adjusting the feet drop (https://www.reddit.com/r/SewingMachinePorn/comments/1fojtgp/1927_excella_made_in_germany/)
if you can find a manual of any machine of your brand, there should be some specification about it
I can't see shit on pic rel