I got one of those pedals (double bass version) gifted second hand.
The previous owner did not detach the beater of the second pedal correctly but let it hang loose. Long story short the thing between the shaft and the beater holder was wasted and unusable.
The last picture shows one of the original parts in working condition.
Even if your cranked it all extremely tight the beater would wobble around.
So I made this replacement instead.

It fits a millenium pro pdeal beater holder (number of grooves matches that) but will probably fit others too. I ended up attaching the beater to a second hand direct drive pedal (see pictures).
I guess the shaft diameters are the same on most pedals.
Anyways you might need some adjusting of x/y-compensation and scale with multiple tries to make it fit your shaft and the beater holder.
If you have to apply reasonable force to get in on the shaft then you probably got it right.

The beater angle can be adjusted with this really nicely even if it does not have a groove in the middle as the original part.

Printed in nylon but really any filament should suffice.