holder-up

One who holds up something.

A bucker; one who bucks rivets, typically holding a heavy bucking bar against the bucktail of a rivet (heated if necessary till it is soft), while the riveter (or gunner or, before mechanisation, basher) uses a rivet gun (an adjustable-impulse pneumatic hammer) fitted with a rivet set, against the factory head to provide impulses which upset the bucktail into a field head.