I've found very little on CN about this eyepiece, I think it dates from the 70's. The threads that couple it to the 2" adapter are the same as the Vixen visual back, 36.4mm, so a 1.25" adapter would probably reduce the outer field illumination a bit, but not objectionably so and maybe not even noticeable.
This eyepiece is huge, that is a standard 1.25" volcano-top 12mm Kellner for comparison. I have not tried it yet.
Any information or user reports are most welcome. Due to it's age, I thought I'd bring it up here first, if it gets no traction in Classics, it can be moved to Eyepieces. Thanks!