I HAVE 3 DISTRIBUTERS FOR YBLOCKS THE OLDEST 1 USES A SMALLER SHAFT FOR ROTER,THE OTHER 2 TAKE THE ROTER WITH LARGER SHAFT.. NO TAGE ON THE DIST FOR SMALLER  ROTER SIZE. 
IF YOU WHER CLOSER I WOULD GIVE YOU A NEW 1. I AM ON VANCOUVER ISLAND BC

Try one of the Tbird suppliers.

I’ll suggest trying the new rotor on another distributor.  From this end, it sounds like the rotor post on your distributor is either undersized or the top is pinched inward slightly making the rotor loose on the shaft.  Got a picture?

Lorena, Texas (South of Waco)

I have a 1957 Ford distributor ( FEK-12127-A ) I believe it is 1957.  I have been to 4 different auto parts stores looking for a rotor.  A seen to be to large and when placed on dist. is loose. I think the 56 and earlier and 57 the clip was done away with.  So way is it so hard to find a rotor that fits?  Any Ideas on what or is the others that had this problem and how did they deal with it? I figure some kind of shimming but wondering what to use Don't what to mess up dist.  Open for your help.  Thanks, a great sight for us that need answers.