Once upon a time, a long time ago in the daze of tubes (remember, 2 WATT carbon AB pots), there were a few pot makers that made "kits" where one could stack one pot above another and drive them with a single solid, selectable shaft. D-shape, knurled, or round. Optional switch at rear. Sort-of like the CTS style. Later Bourns modernized the idea using their newer rectangular box shape and film tracks, but again,they were stackable.
In my junk box, found one (my infamous joke, "can you count to one)?" shaft, 1/8 inch dia 3 3/4 long, split drive both ends, number stamped on it RF8203.
Where can i get these parts, especially those shafts? Mouser, Digikey, baby bird (GooGull) not useful.