Aha, good to know. The mount you see on the lensback of the separate lens is not the Rollei mount. Looks like a flanged-adapter PK conversion.
FWIW, the Rollei HFT Planar 1.8/50 should be something like 10 dollars really. It is the same lens as the older 'Carl Zeiss' version like on the camera on the photo, same lens as 'Voigtländer Color Ultron'. The latter two are available also in M42 mount. Then they are suddenly valued 100 dollars (speaking strictly market prices here and only 'made in Singapore' versions).
The Y/C Contax Planar 1.7/50 is something like the follow-up of this lens, behaves very similar, better coating probably. I'd go with this one, rather easy conversion. But hey, the 1.8/55 S-M-C Takumar can do magic too.
More on the different conversions here,
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/61708-converti...es-pentax.html
Best, Georg (the other)