Sorry Brenda I cannot think of anywhere on the Mark V, Jointer, Belt Sander or Bandsaw that uses that long of a 5/16" set screw. I remember looking at something a while back that has a long set screw, probably an accessory but I cannot remember what. I work on Model 10's a lot but I don't think it was something from them. Those have more 3/8" and 1/4" set screws and very few 5/16" ones and no 5/16" ones that long for any Model 10 or accessory. If it goes to anything Shopsmith maybe someone else will know.
Mark V completely upgraded to Mark 7
Mark V 520
All SPT's & 2 Power Stations
Model 10ER S/N R64000 first one I restored on bench w/ metal ends & retractable casters. Has Speed Changer, Model 4E Jointer, Jig Saw with lamp, a complete set of original accessories & much more.
Model 10E S/N 1077 oldest one I have restored. On bench w/ metal ends & retractable casters.