Wasn't me
I think it must be a drilling jig of some kind because with that thin of a base plate, it's not rigid enough for milling. If it was used for welding, there would be a threaded ground connection or scratch marks somewhere from a grounding clamp. Due to the configuration of the work piece holding clamps, it is specific to one particular component rather than something universal. Close examination might reveal more clues.
Duh
Ryland