Fitter or lead weldor's tool.
Pipefitters use them to set up spools (subassemblies) of flanges, tees and elbows that get built in the shop and trucked to the field for install. Spools cut time by probably half since they can be built in comfort and often on a positioner. 1 Pick with a crane puts a spool in place as opposed to building it piece by piece in the field.
Lead men use them for structural and machine frame assembly, jack the iron into orientation, tack, check and move to the next assembly while lower paid men weld it out. A lot of shop assemblies are built using a 9" level and tape as the only tools involved beyond welding equipment.