The cartridges are genuine APS cartridges and all have been developed and have the square showing.

Looking at the data you linked to, John, it would appear that APS film was in fact slightly smaller than 35mm. It was standard, here in the UK at least, that APS negatives were returned in their cartridges (I used several different processors at various times), unlike standard 35mm film where the negatives were returned in strips of 4-6 exposures (I have those as well). They usually provided a sheet of thumbnail prints which had each picture on the film numbered so if you wanted extra prints, you simply told the processor the numbers of the pictures on the thumbnail sheet you wanted printing.

I have tested scanning from prints on a flatbed scanner and that works well and is probably the best solution where I have prints available. However where all I have is negatives I need some way of getting the negatives out of the APS cannisters so I can "scan" them. I suspect in the end it will be a matter of trying some means of winding out the film and seeing what works. I just want to avoid losing the originals if I can avoid it, but may have to sacrifice a canister or two.