I have had another look at a box marked 'Canada' full of mostly KOdachrome slides. Some are marked 'Made in Canada' with card mounts and dated '63 and '62. Also quite a few marked 'Kodachrome duplicate' and marked 'made in France' with the name of a French ?processing company. The colours are as fresh as new.
I knew that Kodak had a special slide film for copying slides (quite difficult).
My dad had some Kodachrome slides copied by a 'professional photographer' who clearly didn't know about the contrast problem doing this, and the copies have a very dark blue tint and are so contrasty they are useless.
I did try a trial copy of Silverfast and that was very good, but it was (and still is) rather pricy and ties you to one model of scanner.