mcculloch power mac 320, What do these do?

[fuhlenn]

 

[Old2stroke]

That looks like one of those terrible ideas from McCulloch, called a self sharpening chain. It used a ridiculous "barricuda" chain and the knob presses a grinding wheel into it to sharpen it with the chain running. Never worked worth a damn and most saws have had the chain replaced with a conventional one and the sharpening device either ignored or removed.