IBM 1725 SLS - Manual (Page 188)

The Envelope
Feeder
QMS 1725 SLS Print System User's Guide 8-4
1 Turn the printer power switches off and remove the upper
paper cassette.
2 Remove the envelope feeder slot-cover located above the
upper paper cassette slot by pushing it in and then down.
3 Replace the cover of the upper cassette with the special
paper cassette cover.