Thanks Rob! Only took me four months to respond
Work and life keep me busy but this month marks one year with the car!
Since my last post I've had a steady stream of little projects. The car came without much documentation from its restoration (~2000-2010), so in part I've been "baselining" the car bumper to bumper and documenting everything, correcting little bits as I go (and with regular reading of this forum, CRG, and TC, I learn more every day about the nut-and-bolt details of how an 02D RS Z would have rolled off the assembly line).
I'm eventually hoping to document more or less every nut and bolt, with a list of what's correct, what's not, and whether what's not correct will be replaced or left alone. Two examples: I plan to keep it desmogged and I don't mind leaving the shiny valve covers with drippers since it's a CE block. Biggest ticket item to fix in the engine bay: swap the repro booster & MC with the originals that I've got in a box and need to send out for restoration.
First fun project from this summer was to pull the diff cover to check the gears. As I suspected after counting driveshaft revolutions with the wheels up, sure enough this is a 4.56:1 gearset inside the original BV (4.10) housing
Looks like "Zoom" gears from the stamping "41 x 9 PHT 3093" (which I believe is Perfection Hy-Test). This Z was originally bought in Harvard, Il, and later restored in Chicago, so I wonder if the owner went with Zoom gears since they were local (Harvey, Il, at the time) or just because they're awesome
Looks like the original "4-series" gear carrier, the heaviest-duty from GM which would have carried the original 4:10 set. Assembly stamp is A 23 J so Jan 23rd '69 - seems right for 02D?
I also added a posi lube tag to the diff fill plug, adding the period after "diff" to make the repro look more correct (a slight touch from a center drill to deboss then a drop of white paint). While under there I went for 2 qts of fresh Lucas 80W-90 for the posi with a new Fel-Pro gasket and 1.5 qts of fresh Sta-Lube 85W90 GL-4 for the M20.