Historically the 10-50R outlet was considered the high-falutin way to wire a 50a 240 volt circuit because the neutral was full size, and grounds to individual outlets in those days were much smaller than the current-carrying conductors. If the outlet is fed directly from the service entrance panel (which is after all the most common situation) then the neutral wire goes to the same bus the ground wire would, and the two are effectively interchangeable. In those days, the full-sized neutral was thought to be better than the smaller ground, and so people put in 10-50R outlets for things like welders and other 240v 50a loads.
I would suggest that you have a qualified electrician rewire the outlet for 30A 120V with ground and install a suitable outlet. There are enough conductors to do that and provided they are clearly marked and the breaker or fuse is changed there should be no safety-related reason not to do so.
Otherwise, as others have noted above, grounding is problematic and poses a safety hazard.