I bought some second-hand Crank Bros Eggbeater pedals. They came in the original box, apparently with all the bits except the cleats. (That's fine. CB cleats are effectively a consumable.)
One of the bits is a pair of plastic pieces I don't recognize. I'm still working on getting a photo from my phone in a place I can upload it, but in the meantime, please enjoy these ASCII drawings:
___ , , ___ / _ \ |__ |__ / \ ( (_) ) |▓▓ |__ ( * ) \___/ | | \___/ Front Side Side Back Cutaway
Each piece is a plastic disc with a knurled edge. From the disc, a ring is extruded, with threading on the outside of the ring. The ring's inner diameter is 10mm, and the outside is ~13.5mm. The outside diameter seems to be the same as a pedal spindle diameter, but the threading has a different pitch. There's also a very small hole through the centre of the disc, so it can't be used to seal anything.