What exactly does this mean? in a 91 lo5

LTZ should be the luxury model, but would find it hard to believe you have 91 LTZ with an L05 because the L05 was strictly for the 9c1 (1989-1992) and appeared in the LTZ only in 1993.

Check out the following link:

http://www.9c1.com/technical/engineapps.htm

What exactly does this mean? in a 91 lo5

In GM marketing speak LT usually means Luxury Touring.... and the Z is often used by GM to honor Zora Arkus Duntov of Corvette fame.

But officially, it doesn't mean anything literal except that it is a merchandised sport package for the Caprice consisting of trim accoutrements (emblems and such), copcar instrument cluster & matching dash, most (if not all) of the copcar 9C1 suspension, 3.23 (1991-1992) or 3.08 (1993) rear end gearing. The engine was a 305 in 1991-1992, and in 1993 it was the only 350 available in non-police Caprice models.

LTZ models have also been found on the Lumina. HTH.