I did a search for you on Google. I think this is a free download for the manual:
Singer 211g Manual Industral
Someone who has one for sale and could probably help you:
Welcome to Leighton's - USED EQUIPMENT - SINGER Model 211G
Here's something from Singer:
PDF Service Manual:
Hope this helps. This "We have a question about a knob on the top of a sewing machine that I don't know even exists. ?????? " sure as hell doesn't.