First, let me say how impressive it is that you took apart, and put back together!!! a 401. Next level stuff and way beyond where I am.
As to the screw, it would appear it sets in a recess and something moves around it. Maybe a hold down for one of the cams in the stack? That's where I would start. If you can find a parts diagram maybe you could try to match up the screw to something on that.