This part was in a catalog of rigid conduit fittings. It is just like a reducing bushing, threaded inside and out, but it also has a "hex nut" flange one one end so it can be turned with a wrench. What are these normally used for? Another clue-- the part is a Thomas & Betts 1250. Could this be used to enter a box that has a knockout that's a size too big?
--zeb