Jim,

The early Union Fire Arms doubles were sidelocks. The frames were square all the way forward to the hinge pin and the sideplate extended forward covering the hinge pin. The later model Union Arms Co doubles had a rounded frame forward of the barrel flats and were sideplated boxlock actions. They used a cocking hook and barrel lug similar to the Lefever. The small pin should pickup in the slot of the hook once the forend is installed.

PM me if need more info.

John