From the 2007 Standard Catalog of Baseball Cards:
These unique novelties feature the members of the World Champion 1948 Cleveland Indians. The players are pictured on a single frame of 16mm film which has been inserted into slits cut out on one end of the cardboard. The cardboard holders are dark blue and yellow, with blue printing. Fronts have player personal data, career highlights, the player's name in script and identification of the issuer. Backs have a list of "Magic Slides" which could be ordered for 33 cents each. It is likely that a slide-card exists for each of the players on that list, but only those which haave been confirmed to date are checklisted here, in alphabetical order. Complete slide-cards measure 3-3/4" x 2". A plastic baseball-shaped viewer was sold with the slides.
Click here to view a gallery of the 1949 Vis-Ed slide cards set