I would not call it a pipe fitting, because it is not intended for plumbing use.
As you pointed out, it is a foot for a table or bench leg. I bought a table at an Air Force base surplus sale in 1959 or 60 that had legs with those same feet.
I would try a Hotel or Restrauant supply house. They could be fabricated from threaded couplings for rigid conduit with appropriately sized heavy washers welded on them.
Use conduit couplings, not pipe couplings. Conduit threads have the same form and pitch as pipe threads, but they are straight instead of tapered.