according to this article and other posts I've read, yes, the third members are interchangeable..
https://www.stovebolt.com/techtips/a...Ton%20Upgrade/
the prevailing story on the pinion load bolt is that only early 72 had them but later both 52 and 72 had them..
fair price is difficult to estimate for something somewhat difficult to find, especially locally to avoid shipping..
if I found one within an hour drive radius, I'd give 2-300 for good third with locker..
but someone somewhere is probably selling a locker only for $500 and someone else selling a complete axle for $100
best price gauge is what's it worth to you knowing you can't order one from a catalog anywhere
if it helps at all, I'll send you $25 for the plate in the cab if it's actual plate and not a decal and you can get it out without destroying it
good luck!