LBZ, LB7, and LLY are the production codes (RPO) for different versions on the duramax. They way that you tell which engine you have on your truck (yours is a LB7) is to look at the 8th digit of the VIN.
1 = LB7 (2001-2004 model year)
2 = LLY (2004.5-2006 IIRC)
D = LBZ (2006-2007)
6 = LMM (2007-present)
The years may not be exactly right, but they should be close as I can't remember exactly which ones were which....the VIN number is the best way to tell.
THey are the production codes for the engines each time the engine was changed they changed codes.
lb7- the first dmax,
lly second gen(more power, VVT turbo, pvc system, cat converter, external injectors),
lbz - same as lly but more power and beefed up block and internals, lower compresion ratio, intake heater,
lmm- same as lbz with the addition of particulate filter for 08 emissions