Specifically, can I replace a busted JD690B crowd (end) boom with one from a JD690A? Much thanks to the experts for your comment and advice!
Your title suggests boom and your text suggests arm. So looking at the parts pages for both the boom and arm for the A vs B, and it shows that they had different part numbers originally but all sub up to the same part number. So I am thinking yes, they are interchangeable (either the boom or the arm). Obviously before purchasing what you need I would take the necessary measurements to insure it would work. Go here and check for yourself. https://partscatalog.deere.com/jdrc/search
Much thanks sir, and thank you for the link.