DEC 2100 Operator's Manual page 56

7. Adjust the manual feed guides to accept the width of the envelope.
8. Insert a single envelope into the printer until it stops.
— The envelope is automatically fed into the printer.
— The printed envelope is sent to the rear output tray.
2–30 DEClaser 2100 Printer Operator's Guide
Fig 2-32, MLO-004846
Fig 2-33, MLO-004847