I bought a large (used) Quickset tripod and it had this adapter plate mounted on it. I am not exactly sure what it is intended for, but I am no expert either. Can anyone help to fill me in?

Jamie,

It's a tripod to camera adapter. Appears to match Sony design. It would be attached to the tripod, with the quick release lever on the left side of the camera. This piece is only half of the adapter. There would be another fixture attached to the bottom of the camera, allowing the camera to be lowered into the channels, and slid forward to lock. Looks ancient, so may have been used on Sony Hi8 or Betacam. I don't know which model this one is; however, the Sony VCT-14 is similar and is still being made.

Mark