Thanks!
It's hard to measure this thing.
The shaft diameter is 1/4" or possibly as much as 9/32" if you want to be ultra precise.
The groove seems to be maybe 7/32"
The manual calls it 1/4 e-ring, so I'm assuming these things are measured based on the shaft, not the groove, and the ring is measured based on the inner diameter, not including the little bumps