It appears you have gone into Arbor overload. When the Mark V 510 and 505 were released they needed longer arbors for the saw blades and hubs for the sanding disk. When I mention 510 that includes the 505 as it is the same Table System without the floating tables. The Lower Blade Guard for the 510 and later the same one for the 520 and Mark 7 has a longer throat connecting it to the Quill. This lower guard is not to be confused with the 500 dust collection chute which is a stripped down version of the 510, 520 and M7 lower guard and can use steel Sanding Disks with the shorter hub. Now that we are on the Sanding Disk the short hub (1-1/4") is for the 500 or original table system. The longer hub (2-1/16") is for the 510, 520 and M7 tables. This longer hub Sanding Disk can be used with the Mark V 510, 520 & M7 lower blade guard for dust collection. The short hub Sanding Disk cannot be used in a complete 510, 520 & M7 lower blade guard.
As for arbors, only the 1-1/4" and 5/8" Saw Blade arbors differ for the table systems. There have been only three longer arbors specifically for the Mark V 510, 520 and M7. 555130 is for the 1-1/4" arbor hole SS blades and at first there was 555118 for 5/8 " arbor hole blades that was replaced by 555608 that was originally made for the Sawsmith Table Saw. 555118 looks like a longer version of the arbor 555321 for the 500 table. Below is a listing of those Saw Blade Arbors and their overall length.
505511 - Mark V 500 1-1/4" Saw Arbor - 1-5/8"
555130 - Mark V 510/520 1-1/4" Saw Arbor - 2"
555321 - Mark V 500 5/8" Saw Arbor - 2"
555118 - 5/8" Saw Arbor - 510 only - 2-1/4"
555608 - Mark V 510/520 5/8" Saw Arbor - 2-1/4"
I hope this clears it up some for you.
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.