IBM 1800409-001A - Manual (Page 262)

2. On the envelope cassette, set the back and side envelope
stackers (figs. 8.5 and 8.6) to the correct envelope size.
Fig. 8.5 Set the side stackers
Fig. 8.6 Set the rear stacker
Chapter 8
8 - 6 P r i n t e r O p t i o n s