What is the model designation for a 1-1/4 ton AD Chevy?
There was no 1 1/4 ton Chevy. Or at least not until the '60s. even then, they were part of the 40 series, using an HD version of the 30 series IFS. I've only personally seen 2, 1 was a Chevy, the other a GMC, both in the '67-'72 body style.
