This was damaged in shipping. Looking it over I THINK it's a rotary encoder. ALPS seems to have forgotten they ever made the part. Printed on the back is ALPS 242N. Can't find any specs on it.
Picked up an ALPS EC12E2420801 (24 Res, 24 Detent). Wrong part...
Need to measure all the dimensions first next time. The original part is 16mm wide, the EC12 is <13mm. Had to go searching through mouser data sheets to find one with the correct dimensions. Waiting for the new one to show up but it looks like it should work.
Final Update -- Finished! Picked up a Bournes PEC16-4220F-N0024 (24 PPR, 24 Detent) and it works perfectly. I don't know if the original had detent or not, but I like the feel of it.