Yeah, the drum backing plates are long gone... I assume when the previous owner did the original disc brake conversion. This pic I'm attaching of what the calipers bolt to measures .38".

With the adjuster completely screwed in, I still have .25" of play. With a Detroit locker, and original tapered bearings, are the thrust buttons necessary? I read a lot of threads where people didn't worry about broken ones.

Since mine fell out, I assume the retaining pin is broken and I'll need to get a new one if I need to use the thrust buttons? Any idea where to get one?