Brett,
While the overall concept of a replaceable stud might be the same, it sounds like the Jett has a more robust detailed design. The Kay Und Beast 8011 forward shaft is held on by a tiny roll pin and one setscrew thru the drive hub as shown in the picture. For ease of assembly, the hub is a slip fit to the shaft, not a press, so that adds instability to the joint as well. There are several ways that this scheme can come apart on you. The first is trying to use a spinner nut with the stud and the stud makes contact with the pin before the nut bottoms on the prop...and you're D-U-N done. The next is when the engine has been run hard and the setscrew (hardened steel) chews its way into the crank. Then it's good for one more run--maybe--and yer prop departs.
To solve the first failure mode, get rid of the stud and use a hexbolt instead. Make sure the bolt is short enough with the props you are using to never make contact with the roll pin. Or, use some red LocTite on the stud to keep it from creeping into the drive hub as you keep swapping props around during testing. To somewhat mitigate the second issue, grind the end of the setscrew flat and grind a flat on the crank after you have drilled the hub/shaft for the roll pin. You have to ensure alignment, else you are using up some of the material strength just on poor assembly work.
Once I got past these issues, I have run the 8011's for years in Quicky Rat. There is one obscure advantage to the 8011: since the drive hub has a shoulder on it, you have to counterbore all your props out to the molded ID of the APC prop. (7.8x7's are de rigueur). You therefore eliminate any grief from unbalanced props that came from the factory with the hole not centered. Silver lining!
Dave
PS--Air Miniseries showed the picture and plans for a Grmzpf, but he also had a Sprntr and maybe even a Grzula---all designs by one of our former club members here in the SoCal--Don Burke.