WideOpen

Hiya,
Let's sort this out.
Firstly, the right-angle finder does not have, nor need the knurled ring from the camera's viewfinder. The RA finder certainly has its own provision for fitting a rubber eyecup, assuming it has not been modified otherwise; actually it should come with a rubber eyecup, though many people dislike them. The knurled brass ring, missing in your case, is removed from the camera's viewfinder in order to fit the right-angle finder, screwing into the threads where the knurled ring would usually go. When you're finished using the RA-F, unscrew it and replace the knurled ring (I'm sure they can be sourced on eBay, possibly even here on pentaxforums.com), which also includes either a blank or dioptric correction lens (dioptric correction lenses are long-ago a vanished item, but optimetrists can make them).
Garyh