IBM 1725 SLS - Manual (Page 194)

Duplexer
Assembly
QMS 1725 SLS Print System User's Guide 8-10
8 Remove the four screws holding the inner cabinet in place,
and pull out the inner cabinet.
Store the inner cabinet in a safe place in case you ever remove
the duplexing unit from the printer. If you do remove the duplexing
unit from the printer, you must reinstall the inner cabinet.
9 Insert the duplexing unit into the printer as shown.
10 Replace the four screws that were removed in step 8.