Apple LaserWriter 12/640 PS - Manual (Page 209)

Loading transparencies in the multipurpose tray
You can load one sheet of overhead transparency at a time into the
multipurpose tray for manual printing, or you can place a stack of about 40
transparencies in the tray for automatic feed.
To avoid leaving fingerprints (and ensure the best image quality), handle
transparencies by their edge.
IMPORTANT When you print overhead transparencies, Apple recommends you
use the multipurpose tray and use the face-up output tray for delivering
printed transparencies. (This helps avoid paper jams and curling.)
Only use transparencies recommended
for use in laser printers.
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