Oh exalted brain trust of all things airhead,

I have a new oil leak on my 1976 R75/6. I've owned it since new, and have always run the oil about 1/2 qt below full (i.e., halfway between the "1" and "2" marks on the dipstick). Recently, I checked it after being parked for a couple of weeks, and found it to be down to the "1" mark on the dipstick. Shame on me, I know. So I dutifully added 1/2 qt of oil. The next day I went for a short ride (10-15 miles) with a couple of friends. When we stopped, it was dripping a significant amount of oil off of the bottom of the engine. Sigh. A couple of days later I cleaned things up and rolled it back into the garage (never started it). No more drips. Today I sprayed anti-perspirant around to get a good layer of talc. I started it, and ran it for about five minutes at around 2K rpm. And, voila, the leak showed itself. It is coming from a slotted screw head on the LH side of the engine case, directly below the oil pressure sender. I've checked the parts fiche, my old Clymer's manual, and dug around here and on Snowbum's site. I can't find anywhere that hints at what that screw it for. Does anyone know what that screw does, and what a leak there might be telling me? I won't guarantee that I didn't overfill it, but after 40+ years, I'm reasonably sure I've got that process down.

Thanks,
Kevin