This is a plastic spring to preload the Z-axis guide shafts of the Replicator 1 against one side of each endplate.
In a recent teardown/rebuild of my build platform, one of the (several) observed reasons for its wobbliness was the presence of slop in the plywood plate that captures the bottom of each Z-axis guide shaft. On my 'bot there is no such slop in the upper plates... YMMV.
This little doohickey, once printed, may be wedged between the back plate of the printer's housing and the shaft, thus pressing the shaft to one side of its capture plate and, hopefully, eliminating at least one cause for build platform slop.
I printed at 50% fill, 0.2mm layers, 80mm/s (120mm/s travel), 235Â°C extruder, using the marvelous Sailfish Firmware.
Test your printer first to determine the number of springs you need: grab and jiggle each Z-axis guide shaft near the top, then the bottom. If you feel any 'click' or wiggle, add it to the tally. Then print as many as you need. I printed only two, but I'll be watching those upper plates and, if/when they develop slop, I'll print a couple for them too.
Once printed, just wedge this thing in between the back wall and the shaft. I found it easier to put the wood side in place first, then push down to get the shaft side to slide home. It was a pretty tight fit in my case, requiring a bit of force.