Sounds like someone had some race engine stuff going on to keep more oil up top and slower drain back.
Has nothing to do with keeping oil up top or slowing drain back to the pan, it has to do with stuff that obviously predates your ls experience.
This was an old race trick, commonly used in class racing where you wanted to squeeze every last HP out of a setup and were restricted on parts you could use. Served a similar function as a windage tray, bonus being a slight reduction of rotating mass internally at top end rpms.
Sort of sounds like maybe even a stock eliminator mod. Goal there was youd run super tight side clearance on your rods, then less oil in the sump.... give or take the range of three quarts, you'd keep the windage down in the crank case. Not having oil drip on the cam and crank as it passed downwards would have the bonus effect of decreasing windage, and weight...
Motors done up like that would need to be killed or idled down super quick as well or risk running dry on oil. These mods are for class racing where they are looking for every last hp.
Claims on a small block v8 would be as much as 6, 8, maybe even 10 extra rwhp. YMMV.
But based on that being there I'd take a close look at the rest of things inside for wear and heat related to lubrication issues. Oil drains fine to pan via front and rear big holes, but, anyone doing that probably as stated was pushing the assembly hard. Likely on 3 or 3.5qts oil too.
Overkill in chebby where you could almost always just screw in bleed tubes, more suited to a poncho with bigger openings.
I'm guessing it was run higher rpm and they were afraid of oil staying in the pan at the top end....
Just spitballing here.