Weeeellll, today I feel like the world's biggest dumba$$.
I shoulda checked the other cylinders before I inserted my foot in my mouth.
Two of the other cylinders that were near the top of thier stroke, I sprayed some WD-40 down on top of the piston, and then using a thin screwdriver and rag reaching thru the prechamber hole, wiped enough of the soot out of the way to really get a good look at the pistons.
Same exact markings.
As in "star pattern" or "moose antler" markings.
As in I shoulda checked the "modifications to pistons" section of the FSM before I went off the deep end.
What really threw me off was that I thought I was seeing the exact opposite of what the drawings in the FSM were showing - that the area INSIDE the star/moose antler patterns were supposed to be machined down/recessed, and the surrounding area still flat/raised up.
What I finally realized that the piston drawings were showing, and confirmed by looking at my pistons (and poking at them with a screwdriver to confirm I wasn't seeing an optical illusion), is that area inside the star/moose antler pattern on the drawings is still flat/raised, and the surrounding area OUTSIDE the patterns is what has been machined down/recessed.
Making the latter assumption, then the tops of the 3 pistons that I cleaned off match the drawings in the FSM.
Now, it looks as though instead of having a trashed engine, I may have one that someone did an extensive rebuild on sometime in the past - the intake manifold, valves and passages, and prechambers were suprisingly clean - too bloody clean for an engine with 215K miles on it - as in the entire head had been pulled off - and instead of matching the star pattern on the top of the pistons, as you'd expect on a vehicle with a build date of 7/86, they seem to more closely match the "moose antler" pattern they switched over to in mid-88. Either that, or someone swapped in a newer engine at some point.
Comforting thought, but I still feel like a prize idiot over that initial mistake.