Fm - I am disagreeing with both those guys in regards to the Thomas being modified. The head is removable, very tight tolerance, and can be used as a hand vise. The vise is tightened or loosened by rotating the small lever attached at the back of the vise. Again it open and closes with very tight tolerances. The vise has 4 small cups that align with a guide pin on the stem. You can place the vise, on the stem, in four positions (0°, 90°, 180°, and 270°). The guide pin is not completely round and is made to fit the cups. The end of the stem does not show cut or grind marks.
I know Thomas made and sold vises in the 30s but can’t find any catalogs or reference materials.