LOL....the explanation I was given was that machine owners could understand engines being different by serial break, but truckers needed a new designation because all they could understand is  "3176 no good" 

We've lucked out...we have a 2YG and a 7LG in trucks both north of a million miles, and a bunch of them in Challengers...the equipment uses different prefixes but we have both spacer plate and integral deck versions in the tractors.   You're accounting of years makes me want to go look at ours...because we have three 75Cs and I know that one has a spacerplate but according to the years you listed, the newest should be integral deck...would certainly not be the first time Cat built machines with major differences and put the same model number on the side.