This is an enhanced bearing shim for Carl's AluCarriage upgrade for printing Nylon and other high temperature materials on the Replicator 2. This shim provides something for the grub screws to push against to hold the carriage in place.
Best to print this in Nylon or ABS. The shims should be installed facing the same direction i.e. with the taller edge aligned with the rear of each block - this will ensure the carriage is centerd and friction free. I recommend using Locktite 243 (blue) to hold the grub screws in place - the just need to be firm enough to seat the bearings and prevent play (not tight), and nothing more.