and they look more like seals from a trade or manufacturers' association, as opposed to a government-issued stamp.
I'd say you are probably on the right track, Dave. The bottom arc of the bottom stamp translates (at least in Chinese) roughly to:
Japanese Plywood(?)Industry Union
Hopefully, tonymacg, PostmasterGS... will chime in with the correct translation and everybody can have a chuckle at my expense.