ya very weird. I tried looking up pictures of the weird lifter bore but i think i lost them.i did find my engine all torn apart at the machine shop which would have been the same day. we removed oil pan and rigged up a hose to a junk pickup tube ran to 5 gal pale with oil in it. then primed the engine and saw right away that the valley was filling up with oil.I have seen some weird stuff in 87 as well. The 87 G20 I have was a very early build. The stock TBI 305 had 187 heads with the port walls milled between intake ports like the 4.3 heads with stainless steel clips pressed over them. Some of those clips had actually broken and the ends were missing on them, I am assuming the engine injested them although it ran fine.
I also discovered despite having a catalytic converter, it was an open loop TBI system with no provision for an 02 sensor and no EGR valve. The intake manifold was cast shut where the EGR went. It also had an OE roller cam and torx bit valve cover bolts.
That being said almost every TBI engine I have been into had self-aligning factory rocker arms.
My parents 1980 C10s 305 had the lifter bore machining messed up from the factory. Several of the lifter bores were factory sleeved. When that engine got up over 100K the sleeves pushed out and the oil pressure dropped. Our mechanic at the time, shoved them back into place and tack welded them to the block. That repair worked a good number of years until dad had a low mileage 350 swapped into the truck.