As mentioned before there are currently two Shopsmith 5/8" Arbors. One, part number 555608, is longer for the Mark 7 and Mark V 520, 510 & 505 that have the larger table system and is needed to fit their lower blade guard. The other is shorter, P/N 555321, for the original Mark 5 and Mark V 500 table system. I recommend you get this one for your Mark V 500. Using the arbor made for the original table system like yours makes set up simpler as the carriage stop and blade guards are designed for it. If you have Lower Blade Guard P/N 505628 the longer 5/8" Arbor P/N 555608 will put the blade just outside the guard unless it has the width extension. I do not know if there is any problem using the later Mark V 500 Blade Guard assembly P/N 505875 with the longer Arbor P/N 555608.
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.