Bob
That was helpful - I determined the UDM (at least the version I have) is not a PC 7424 clone. The counter weight design is not constructed in the same manner.
After googling some more, it appears that it is a c clip that came apart. Looks like a replacement c clip can be purchased at any...
I had this happen to me recently on my UDM.
Ultimate Detailing Machine™ Dual Action Orbital Polisher with FREE BONUS!
It looks like the snap ring broke, and now the inner piece that the backing plate screws into can slide around. The entire backing plate actually flew off while I was buffing...