Both the BVM and Dynamax fans use a 1/4" allen bolt for the probe to engage, so both manufacturer's probes have a 1/4" hex ball. The difference is in the diameter of the shaft of the starter probes.

The BVM probe is made out of 1/4" steel rod with brass tubing on the outside as a handle. I would guess the outside diameter is no more than 5/16".
The JMP probe is machined out of much larger stock which gives it a much larger outside diameter (I think it was bigger than 1/2").

The JMP probe is beefy and I doubt one of them has ever broken, and I think the ball ends are replaceable with a couple of screws.
The BVM probe has the ball end silver soldered into a brass sleeve, and they have been known to break as a result of improper starting technique. The ends can be replaced, but it requires a torch and silver solder.

The JMP probe could not be used on many BVM models because it was too large to fit through the access hole.

I made my own that were similar in design the the BVM one, but used 5/16" steel rod instead. I soldered the hex ball on with brass tubing, and threaded the other end so I could install a standard model spinner. It took less than an hour to make one, and it saved me a lot of money.