Thank you, Robbie [230624 MMD]! Actually I own the remaining stock
and archive of the Riley-Sloan Company. It is almost certain that,
in the 1920s, it would have been 4" x 3-1/4" glass lantern slides
that were used with the Aeolian Duo-Scope system.

In the Riley-Sloan archive I have now found lists of all the images
used with each roll; there are sometimes as many as 20 or more slides
to be projected in the time taken for the roll to play. As the Riley
company stocked many thousands of slides for rent or purchase it is
most unlikely I will be able to find all, or even any, of the slides
originally used, though I do have around 1000 of their remaining
stock of lantern slides.

The company did make a few movie films in the late 1920s and were
pioneers in the film strip business, stocking them from around the
Second World War up into the 1960s.

Regards,
David Evans
British Columbia, Canada