jjj240,
As JPG already said the 555572 Arbor is for 1-1/4" arbor hole saw blades only. If you refer to the picture I posted of the 555608 and 555572 Arbors, you can see the 1-1/4" raised area around the threaded end. The picture below shows the common 1-1/4" Arbors for the Mark series tools. The longer one on the left is Arbor P/N 555130 for the Mark V 510/505, 520, Mark 7 and Mark 4. The Arbor 555572 can be used in place of that one. The one on the right is Arbor P/N 505511 for the Mark V 500 and the original Mark 5 Table System. 505511 can also be used for the Mark 2 and the original Mark VII (not the current Mark 7) made in the 1960's.
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A little history on the 505511. It dates back to the introduction of the Mark 5 in 1954 and at the time was part number 22 032. In 1964 all the parts numbers changed and that Arbor became 505511. In addition, around the time the part numbers changed Magna began putting the black protective coating on bare metal parts. The 505511 Arbor shown above is a later version having the beveled top while the earlier versions had a rounded top. The picture below, although incomplete, shows many of the Arbors made for the Shopsmith Tools going back to the beginning in 1947.
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