Newbie here with a question. I have a 76 Jimmy 4x4 400 with a RPO listed M40 automatic transmission. If I understand some of what I learned here the M40 is a TH400 trans. How do you explain the fact the truck actually has a TH350 automatic trans. I'm sure the truck is all original but one of the three must be wrong, the RPO code, trans, or me (probably the latter)?