First off I need to say that I have not researched all the 80 parts manuals to prove this but from my experience the answer may be the in the base.
The Wayne 80 uses a stamped steel base that has tabs that stick up to fit inside the frame rail. The bolts went thru horizontally or sideways.
The other style of base used a frame that had a plate welded on the bottom so the mounting bolts went straight down into the base. There are several variations of this base; cast iron, cast aluminium, stamped inverted. I believe all these variations were earlier.
Maybe Jack could weigh in, I've wondered this myself but never knew for sure.