Good Morning, Photofox,
Yes, it was a kind of double slide mount. Imagine two standard slide mounts in horizontal aspect, one above the other, but with just a narrow frame space between the openings. I think I cut mine from black cardboard. The whole thing was probably about two inches wide by maybe three inches high.
Once I established the correct orientation of the original slides in relation to the copy film in the camera, the procedure involved dropping in slides #1 and #2, making the copy exposure, then repeating the process for #3 and #4, etc. It was just a matter of being organized and not losing concentration; shooting the same pair twice or leaving out a pair would have ruined the whole filmstrip. It turned out OK and seemed a lot easier to me than trying to shoot forty to sixty perfectly exposed, perfectly framed, perfectly focused, and perfectly organized half-frame shots directly in-camera.
Konical