Just more one possible pattern to toss in there..... My '75 1900 was initially sold in WA state. Oliper reports no digits difference and he is in San Francisco. So those are 3 west coast cars with no digit difference in the different tags. (My '72 Manta was initially sold in Germany so is not a US/Buick imported car.)
Now switch to the cars with the single digit difference: All are owned on the east coast. IIRC, from my waaaaay-in-past recollections, the eastern port of entry for these Opels was Baltimore. I dunno where they came in for the West Coast but it was not through Baltimore. It may be that the east coast cars came from one factory and the west coast cars from a different factory....????