I don't know about the letters from LMC, but the ones I saw on ebay were all grouped together in one 67-72 category. That lead me to think they were all the same for the different trucks in that generation. I know it's not the best source for information, but they fit just fine on my 72 tailgate, and they look great too.
Edit: just noticed you want to compare the 54-66 to the 67-72. I don't know about those
