You might talk to the folks who make the restoration batteries - they have them. Production batteries had the yellow caps, and retail batteries had the red caps.
Talked to one person at Fall Carlisle who sells repop batteries and he told me he did not know where to find the yellow caps. Completed searches and have been unable to find anyone who repops the Y55. Only Y55 I found in searching was the NOS one Heartbeat City has. I have the hard part (battery) but need those yellow ring caps. Guess I'll be painting the red ring caps some day. Anyone have information?