Master Cylinder push rod bolt oring size

I bought just the rubber bushing, but forgot where. Probably Dr Diff, Booster Dewey, or one of those brake specialists. I recall the first bushing came with the 2-4 bolt firewall adapter plate I bought, but I destroyed it when I test fit the brake rod into a later MC piston (99 Breeze). I tried to remove the rod to install a rubber dust shield and tore up the bushing getting it out. That MC is for power brakes so isn't supposed to have the mentioned locking groove in the piston hole, but the rubber seems to grab well regardless.