Mike, I haven't seen any reference for the model #s. I was using logic that since the 57 used 327 and 328 numbers and 59 used 347, and I would think 348, that 58 would use 337 and 338. It seems they went in increments of 10 with each year.
I emailed GM for help with this question and I was sent PDF files of information on specs, but nothing about horn models. I'd like to find somebody that has a 58 and 59 with both horns and see what their model #s are.
