This is an extruder spacer replacement part for the Thing-O-Matic.
When I was putting my Thing-O-Matic together, I was confused as to what the "extruder spacers" were, and as such had to tie the extruder to the platform with pipe cleaners. When a helpful Makerbot employee showed me the correct part, I discovered I had only 1 of them!
That then led to this part, which I managed to design in openSCAD and print out. It is now firmly attaching my extruder to its platform, and has lasted for many prints.
After this was glued on, I of course found the other acrylic spacer.