a.k.a. follower blocks, compression plates, back plates
It's the thing in the back of the file cabinet drawer used to cinch the files against the drawer front to keep them from falling over or from emptying their contents, especially when the file cabinet is being transported.
These heavy-duty plates are for letter-size filing cabinet drawers.
Fits a standard 1-5/16" interior track on either side of the drawer.
Full width tab-to-tab is 12-7/8" at the front and 12-13/16" at the rear.
Each has a spring that attaches to the back, with tiny aluminum wheels that run along the track inside the drawer. One set of those wheels is missing, but not strictly necessary to the operation.