Good "tip" Paul - thanks.
Here is the exploded from the CASCO catalog. It has the 2142 marked. Just as Paul said - it goes on the end of the pushrod. It looks to me as it would then fit in the tapered hole in the MC.
Pg 10 in the catalog.
2142 Plastic Tip - Master Cylinder Push Rod....................7.09
Six bucks at Tee Bird Products in PA. Probably others also.