[Schurkey]

Another benefit to the hole at the rear of the gallery, ahead of the distributor, is that by eliminating a "dead-end" in the oil system, there's reduced sludge build-up.

Oldsmobile 330--455 engines have essentially the same oiling system as Pontiac. I bought a batch of 425 and 455 Olds cores during my "Toronado" phase. Most had holes in the plug ahead of the distributor gear, at least one did not. The core engine with no hole in that plug, had the oil gallery filled with sludge to the rear lifter. Promoting oil flow in what would otherwise be a blocked passage allows the particulates to "escape" into the oil pan, hopefully they're caught by the oil filter on their next trip past the oil pump.

The front of the oil gallery on the right-side lifter bank which gets the air-escape hole in it's plug isn't a dead end; it feeds the front main bearing, the front cam bearing, and the left-side lifter gallery. The left-side gallery gets a hole at the rear, which happens to spray the distributor gear--and I like that concept--but it also allows the sludge out.