I recently dismantled the lens assembly on my old Seek XR so I can add some pictorial detail to Bill W's description.
The first image is a view of the inside of the lid of my old XR Seek unit. I labelled the end of the thing I call a blocking pin. I call it that because it serves 2 purposes--it prevents you from turning the lens more than 1 turn and it prevents you from unscrewing the bayonet lens mount to remove it from the lid of the camera. I had been wondering if it had anything to do with either of those things since I was trying to remove the lens from my old Seek XR in order to put a 4" focal length ZnSe lens on it. I hate to force things but I finally pinched the bifurcated tip of that pin together & pushed it all the way into the lens assembly. After that I could turn the lens several turns & when I forced it beyond its full CCW travel it unscrewed from its bayonet mount! Until that time I had not realized that there was a bayonet mount. A more astute observer might have reasoned that from those curved slots in the cameras lid, but I missed it.
The second image shows the lens assembly parts separated. I gave the parts my own names, having no knowledge of what the official names might be. The lens is held in the threaded holder by 2 prongs, one of which you can see in the photo, but the lens did not simply drop out when I pulled the prongs away, so I did not bother pushing it out just for the sake of showing it apart. The outer shell has 3 pairs of fins that mesh with indentations in the lens mount in order to turn it. One of those fins is taller than the others and is what hits the blocking pin to limit the focus range. The outer shell also has 3 tabs which protrude inward just enough to clip the shell to the bayonet ring. The bayonet ring is metal on the old XR but is plastic on the newer non-XR unit I bought in January.
With that blocking pin removed, the camera can be focused as close as 2 inches. So if you don't need any closer focus than that, you don't need an additional "bifocal" lens.
The non-XR lens assembly is similar except that the outer shell is less than half the height of the XR shell and there are posts instead of fins to turn the lens. The third & fourth images show the non-XR lens with the outer shell off. I trimmed off the top of the blocking pin on that camera & was able to focus it down to about 1 inch.
The outer shell came off the non-XR rather easily just by pushing tweezers under it and prying it off the bayonet ring, but I have not had that luck with the XR lens. I have to pull the tabs away from the bayonet ring one at a time & push the shell off the ring partway at each tab--AFTER removing it from the camera's lid. I suspect that the proper way to remove the lens is to pry that shell off first, then remove the blocking pin, and then unscrew the bayonet. I expect Seek has a couple special tools for that purpose but I am stuck with tweezers & pliers. The method I described above that I initially used to remove the XR lens is likely not "kosher", as there is a risk of breaking those fins. In fact, after a couple times of using that technique it did not unscrew the bayonet but the fins started ratcheting on the lens mount as the lens jammed against the top of the shell & started forcing the shell off of the bayonet ring. Maybe that's still the way to go then, since getting the shell off is what I suspect is the proper first step.
The thread diameter & pitch of this lens mount are the same as that of the lenses on the cheap web cams & "security" cameras I have. This should facilitate making a lens mount for a telephoto thermal lens.