Quoted:
Just some guesses I'm throwing out there...
The part in the center of the mag that looks like a flat top cotter pin,
Looks like it may rotate 90 degrees counter clockwise to allow keeper to come off releasing the assembly.
Thanks for the tip. That part on the top is riveted; can't remove it without destroying it. I went a different route. Since my goal was not necessarily to remove the spindle, but to render the drum incapable of feeding ammunition, I found a workaround that accomplishes this, and I'll post that here to help anyone else who comes along in the future trying to solve the same problem.
I started by poking a small screwdriver into the hole on the front, and rotating that knurled knob until it popped out.
This released the knob, allowing me to pull off the winding lever and internal springs.
Now the center wheel turns but cannot engage the spring, so it can not feed ammunition.