From your description I am not sure if you have an older or newer Scroll Saw. If older, there is an easier way to change blades with the Blade Block Upgrade as seen at the link below. If you already have this type of Blade Blocks than the Mounting Fixture is not needed.https://www.shopsmith.com/ownersite/catalog/scu_bladeblocks.htm
The Blade Mounting Fixture for the old/original Blade Blocks is on the edge of the table as shown in the picture below.
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.