70 GMC VIN codes

[94gmc454]

It appears that GM used a different code system for VINs on GMCs than they did with Chevys for the 67-72 models. I have found plenty of data to decode Chevy VINs, but can't find anything anywhere that decodes the VIN data for a GMC. I've almost got it figured out, comparing it with Chevy data information, but I'm stuck somewhere in the middle. My VIN starts out with CE134, which I believe means C=2wd (conventional), E=V8 engine, 1 means half ton pickup, and 34 is cab and Fleetside pickup box. Then there's a hyphen (-) and 2 letters, BZ, followed by the production numbers. Does anyone know what these 2 letters (BZ) stand for? I'm guessing that one might be production year and the other is assembly plant. I'm having the same problem with the model code CE15934. The 5 and 9 have me baffled. And I've never seen a hyphen in any VIN ever before. I have to admit that I like the fact that it's different from all the rest. Any help here would be greatly appreciated.