Forgot's to update's you's all's (or maybe it's in the Van thread but who cares).
GM original part number is 24503492 - of course it's discontinued. The description is 0.864", other dimensions I could find are 0.1x1.1x1.1
The stretched out O-ring measures close enough that 0.1" is the cross section (2.54mm). Since it's stretched out, the original cross section would have been slightly thicker than this (0.103/2.62mm is the actual measurement of the "0.1/2.5mm" standard).
The vacuum tee OD (the seal ID fit) is 0.868" (22mm). The hole in the supercharger case is 1". The supercharger is a counterbored hole, 0.377" deep (9.6mm). The shoulder on the vac tee is 0.232" deep (5.9mm)
Based on the measurements, this is an axial seal oring with 10% compression.
The closest standard size is -118 (7/8 ID, 1-1/16" OD, 3/32" CS).
Since this is a vacuum tee oring, it only has to seal against air and moisture and heat, right? Wrong, that's why they are stretching out like hell. The oring appears to be EPDM - I say that based on results, since there doesn't appear to be an industry standard for colorant in the material. Usually, black=EPDM, brown=nitrile, green=HNBR, blue=Flourosilicone, but that doesn't apply to automotive where O-rings can be color coated as an assembly aid and to save cost no colorant added to the base stock, and the color coating can peel off or ooze out over time... Anyhow, EPDM is chemically incompatible with motor oil, which it is exposed to thanks to the PCV system, and swells (expands, stretches, etc) beyond usable. It's fair to note at this point that "chemically compatible" is relative
and is graded on a scale. EPDM fits the symptoms. Also, when I say chemically compatible o-rings are HNBR, viton, and flourosilicone, I mean that they have less than 10% swell and are within the recommended temperature ranges.
Of course, I grabbed a 10 pack of -118 flourosilicone O-rings, because they are the same size used elsewhere (like heater hose elbows).