I just measured the inner diameter of the threaded eyepiece mount of my ancient Kowa TS-9 - it also had 27.65 mm or so - very fine pitch thread. I only have a 20x fixed EP for it, but 11-33x zooms are around. All the threaded EPs of the old Kowa single digit TS series should be the same (TS-1 to TS-9).
Some speculation exists that some old EPs for Opticron and other scopes might fit the Kowa too - see https://www.birdforum.net/showthread.php?t=58475
Beware of the single digit TSN-1/2/3/4 models - they're great but these take modern style small body bayonet EPs - as do the TS-611/2/3/4.