I saw a MK III Trooper barrel on a MK V frame (and MK V pony) not too long ago - the mismatch caught my attention as I always notice the MK V Ponies with the "V" in it.
Anyway, this gun had been treated really bad, kind of like it was an illegitimate stepchild. Whomever owned it sure mistreated it -- on the other hand, the dealer's table looked like a real scrap pile.