Bottom line we worked out the size and wood and I cut two dozen 100mm disks out of Baltic Birch ply.
I'm always amazed at the uses for holdfasts. The sanding block is one of those "point of sale" gadgets I'm a sucker for. The good news it is very handy, it holds new or used 5" sanding disks with hook and loop. My guess is it is used for 90% of my hand sanding.
My understanding is the disks will be painted different colors and used in some kind of "matching" game. Whatever, I got my Ataboy for the day.