[Rocky Rotella]

Well, the Engine Unit Number (EUN) or Motor Unit Number (MUN) are general (and accepted) terms that refer to the Engine Serial Number (ESN), which is what Pontiac technically called the engine's sequential build number.

In the example of your 1967 block cast during 1970 model year, there were certain instances, and I don't know the reason, that Pontiac went back and revived a former casting. It may have been by special request by someone within the foundry or division or to fill specific criteria that the current production casting didn't possess. I don't think we'll ever know the reason.

I've also noticed that the SR8 number (which signifies Service Replacement 8-cylinder) is progressive. Blocks in the early-60s were in the 3000s. By the mid-70s, they were approaching or had surpassed the 9000s. It's my understanding that the SR8 number was similar to the ESN on the block and that no two were alike. If that's true, was it possible that Pontiac had only produced some 10,000 (or so) SR blocks in the 26 years its V8 was in production? It sure seems that way.