Hi Bink - no, I think someone must have either dropped it on that corner, or lifted it by the posts, the end one is completely shot, plastic molding & knob broken - the MM/MC knob (diagonally opposite) is also damaged internally, but hopefully repairable with epoxy. It would have been a quick easy fix if anyone had any posts laying about, but it's a straightforward job to make a new back plate and mount some decent posts - as you say, they're not the best (I have similar on both a TA-F500ES and a TA-F530ES, but they're pcb mounted, and not so easy to change without a good reason).
I'm still very much the newbie, so slowly working my way through the amp, looking for faults / parts needing replacement, and studying each section as I go.... I much prefer to get it up & running properly before a full strip, clean, and (probably) recap., I'm not a fan of the 'shotgun' approach, I spent too may years in research for that !
Anyway, thanks again.