If I would have turned the arbors over for the picture it says "Out for Left Spindle" on the other side. It's the same arbor for either side of the power head just keep the proper side out. That helps keep the nut from loosening as the spindle is turning against it. They come up for sale on ebay but rarely. Remember there were only 25,406 SawSmith Radial Arm Saws sold during the 10 years they were in production. Since yours was made by Magna American it would be 1963 or later.
Mark V 520 S/N 09-11-01 completely upgraded to Mark 7, with all SPT's & more.
Model 10ER S/N R64000 first one I restored on bench w/ Shopsmith 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/ Shopsmith metal ends & retractable casters.