It sounds like someone modified, custom made or they came from another source. JPG suggested the Saw Smith Table Saw. The Standard and Pro Models had the same tubes as the original Mark V 510/505 tables using screws and spacers to hold the tubes to the table instead of studs. I do not know the length of the tubes. The Standard uses one set and the Pro has two sets for a wider table. The other two Saw Smith Table Saws had a rail and fence system similar to but different than the Mark V 520.
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.