Rollei lens as Zeiss or vice versa...or what?

At the risk of upsetting the seller, I have serious doubts that this HFT labeled C/Y lens was a prototype, and is more likely an assumption of the seller, and not a documented fact. A simple search got me the listing I think you're referencing, and with a second search for HFT 28mm, I found another listing for a Rollei model in QBM mount, as a "Rolleiflex SL35 2/28mm Distagon HFT", that has also been described as a " Prototype". (www.ebay.com/itm/266383222031)

These are not from the same seller, BTW, so I can't say there's any intent in mislabeling or misleading either as anything other than production models to 'enhance' their value in some way... however... the Rollei branded 2/28mm has what looks to be the exact same front lens ring as the 'prototyped' C/Y version. One other similarity is the serial numbers, with the extremely close final digits, with 2593912 on the Rollei vs., 2593909 on the C/Y.

My guess is that there was a repair done long after any remaining spare parts from Yashica were available, and the Rollei ring was a drop-in fit to make it a suitable replacement in a production model Zeiss Contax in C/Y mount -or- someone lost the original ring for some reason, and wanted a replacement... then (someone) found the Rollei was a perfect match.

I'll leave the Rollei len's status as a 'Prototype' to someone familiar the brand, but the pricing between the two listings seem out of whack no matter how they shake out.

With some searches done, here's at least a few references to go on...

If the numbers on Camera-Wiki are somewhat correct, there was a standard production run of Rollei 2/28mm Distagon lenses of under 1000 units. That's not to say there wasn't a prototype or three, but it doesn't seem to indicate that all Rollei 2/28's were prototypes, and then the production runs were made up of the 2.8/28's. Nothing there to help with how to identify what a prototype would be, either, but it does offer some details on diagramed groups/elements.

Lens-DB has images and specs on Rollei 2/28's, and notes they were announced in 1982 in QBM mount for use on the SL35 line. The filter is detailed as having 55mm threads, so the ring could potentially be a drop-in fit to the Zeiss C/Y version... assuming the inner diameter also matched for lens clearance -or- it could be milled to the same size.

Apparently, there were 3 different 'Pin Types' for QBM mount lenses. That has me wondering if the QBM model I'd linked to earlier could have been considered a prototype when it is pictured as either a later 2 pin or 3 pin model. I honestly don't know if every mount modification would require new prototypes for each and every change, and again, I'd only be guessing about how Rollei works in new designs.

One last tidbit for now that I haven't yet tried to find details on, is what distinctions there are between the hyphenated "Rollei-HFT" and the Zeiss branded "HFT" lenses in QBM mount.