This whole reticle eyepiece thing is kinda frustrating. Seems to be plenty of demand, yet oddly very few are in production.
The Bushnell Spacemaster has been around forever and has a pretty big following, yet no reticle eyepieces exist. Same with Kowa, there are lots and lots of Kowas being used by tactical and competitive shooters all over the world, but Kowa has no interest in making a reticle eyepiece.
The brands that produce and sell quality glass in a reticle equipped spotting scope are all pretty highly regarded, and get away with charging a premium. The Spotter 60/45 and 4000.00, The STR-6 and STR-8 at 3500.00, Optolyth at 2500.00, Leupold at 2000.00
Many people buy spotting scopes with lower end glass just for the reticle. The Bushnell FLP, Bushnell Legend T Series, and much lower quality glass like Barska, NC Star, etc.
So, one of the only companies shooters actually like, making a reticle eyepiece, Vortex, discontinues theirs. What gives? A Chinese lens supplier go under or something?
I saw a Burris branded milrad eyepiece in the 250.00 range, and the scope it fits looks pretty nice but is only available with an angled eyepiece. And even then, your still paying 1500 for the scope and 250 for the eyepiece. In the same range as Optolyth and Leupold.
I wonder, if someone machined an adapter to allow a Burris eyepiece to be fitted into a Kowa TSN-4, if the reticle subtensions would be accurate. Or perhaps to allow an Optolyth eyepiece to be mounted on a TSN-4, or maybe turn and thread the Burris eyepiece to fit in an old Bushnell Spacemaster.
I own a machine shop equipped with both manual and CNC equipment. I could probably alter an eyepiece, fabricate an adapter, or some combination of the two if the reticle subtensions would be correct.