Open up the Organizer (via Navigator). From the left hand panel, Browse to the file where you want to import the Master from. Select the Layout Book icon, and drag the desired Master into the active file in the right hand panel.
MacBook Pro Apple M2 Max, 96 GB of RAM
AC27 US (5003) on Mac OS Ventura 13.6.2
Started on AC4.0 in 91/92/93; full-time user since AC8.1 in 2004