The inverted cup on the bottom of the 2500

I am wondering if that little cup on the bottom of the mpc 2500, with the hole in the centre serves any purpose.

a screw is placed there when the cdm25 is installed, it holds the drive in place.

yes, and that little screw is included in the cd-m25 package if you buy one.
the four little srcews included in your mpc are for the mounting of a harddrive in the hdm-10 adapter which sits inside your mpc.

peace jo