RFGuy,
First thank you for linking the two videos. I had seen Scott's (obviously) and didn't recall it. I had not seen Doug Reid's before.
A bit of clarification, in case anyone missed it, is the Mark 5 originally used 9" Saw Blades. The optional Saw Blade Guard (lower part) couldn't use larger than a 9" Blade. It wasn't until the 1960's that 10" Saw Blades were used for Shopsmith tools.
Now I apologize for the misinformation in my Arbor Wrench post. The Base of the Arbors, until recently, use a 7/8" Wrench, not the narrower open end of the Shopsmith Arbor Wrench as I previously said.

That end is for the 5/8" Saw Blade Arbors nut. Now the "until recently" when the latest version of the Molder/Dado Arbor (previously the 5/8" Universal Arbor) P/N 505506 and the last version of the 1/2" Universal Arbor P/N 505505 came out with a larger diameter base and use a 1" Wrench. The 1/2" Universal Arbor as it had existed was discontinued and the Spacer & Washers were replaced by the Shaper Rub Collars with the larger diameter Base becoming the Shaper Spindle P/N 555117 which was a use for the 1/2" Universal Arbor.
Here is how I made my error, the Arbor Wrench end at the bottom right of the picture in my previous post has a narrower opening then it should have. Checking this morning I found that Wrench actually is too narrow to fit on the 5/8" Saw Blade Arbors nut. I compared both of those Arbor Wrenches I have with Arbor going back to the Model 10's and through current Arbors. These Wrenches are not the strongest wrench to be found and can bend under heavy use. I believe that is what happened to that Wrench and it was "squeezed" back to restore its original opening width but went too far. That Wrench is too tight to use on a 5/8" Arbor nut but still a little bit loose for the base end.
I did add a correction note to that previous Arbor Wrench post.
Now I can go eat my lunch of crow.