IBM 1800409-001A - Manual (Page 264)

4. Lift the stacker arm and load 30 or fewer envelopes
printing-side up, so that the flap side will be on the left when
the envelope is pulled into the printer (fig. 8.8).
5. Lower the stacker arm.
Fig. 8.8 Load the envelopes
Chapter 8
8 - 8 P r i n t e r O p t i o n s