I believe that the idea of APS slides was that the design of new models of cameras could allow details of the shots to be recorded on the magnetic coating of the film, and the lab would print this info onto the slide mount. (And possibly that a few seconds of sound commentary could be recorded onto each....though I may have dreamt that bit.
). The panorama function would not work with slide film, so they would therefor be a full frame size to fit into a standard 2x2mount.
Obviously it was a clever idea which never progressed, due to the falling interest in slides for everyday snapshots, and the advent of d****al.
The prepaid envelope is for the Fuji lab in Germany. I phoned Fuji UK who confirmed that the service was discontinued, but suggested that a dip-and-dunk E6 lab should be able to handle the film. Of course, with a width of 24mm, slide mounts are the next problem, but I could scan the strip to make prints.
(Why is it that these quaint by-ways and dead-ends of photography have an odd fascination?
)