35mm(?) slide transparency mount question.

Going through my Dad's slide collection I noticed that he has a number of
transparencies whose outside dimensions match those of the usual 35mm
transparency slide mounts and fit a regular 35mm projector but whose inside
diameter is greater than that of 35mm film...

Apparently from 1960-1963 before he used Kodak developed slide film returned
in their own slide mounts those early transparencies seemed to be a larger
format than 35mm - he tells me they used to return the developed film and he
used to mount them himself in brown cardboard slide mounts.

The overall dimensions are identical to the standard 35mm slide mounts @
50x50mm (2" square) - it is just the 'window' in the middle that is larger
with the result that the 'frame' around the transparency is slimmer.

Would anyone happen to know what type/size/format those early transparencies
might be and if slide mounts might still be available for them?

The film itself is @ 43x43mm - in those days that would equate to @1 3/4"
square

The cardboard mounts are coming apart and will probably jam up an
auto-loading mechanism so I would like to mount them again but standard 35mm
slide mounts would necessitate cutting the transparency to fit - this seems
such a waste and the slides are good quality and framed nicely - chopping
them up to fit seems like sacrilege...

--
Thanks & regards,
-pp-