The official push-on Rollei cap shows up now and then on eBay. This lens and many of the Zeiss and Schneider lenses for the Rolleiflex SL 35 cameras have a 49mm filter thread.
You can buy generic lens caps in that size for a couple of dollars. I wouldn't pay a premium for the official Rollei cap. I think some guy wants $30 or some outrageous amount. I know he wants $250 for the hood for the 25mm Distagon. But nearly all of his prices are major ripoffs. Sorry, getting off track.
The nice thing is that you can buy a single set of 49mm filters and use them on many of the Zeiss or Schneider lenses. I think the Rolleinar lenses use a different thread size.
So, a 49mm filter will fit the 25/2.5, 28/2.8, 35/2.8, 50/1.8, 50/1.4, 85mm Sonnar and 135mm Tele-Tessar.