This is a back for a 4A Century Studio Camera. It snaps onto the back of the big studio camera and then other ground glass backs go onto it. 8X10, 6.5X8.5, 5X7, 4X5 and whatever else you've got.
From my experience the backs that fit Kodak and Century and Folmer & Schwing / Eastman cameras will fit this back fine. I tried my Kodak 2D back in this and it fit well.
This weighs about 6 - 7 lbs with packing and sadly, it does not fit into a flat rate medium box, so plan on a bit of shipping cost to get it to you.
These don't come up too often, so if you've managed to buy a big camera and the back is missing, this might be a welcome find. $85 + shipping.