IF memory serves me right, the GMT hand is set to move at half the speed of the hour hand and you use the OUTER bezel 24 hour markings to read the time
also to set the GMT hand you pull the crown out to the first position where you generally set the date and and turn it the other way. Be sure to do it when the hour/minute hands are at an even keel - 5 o clock, 6 o clock, 7 o clock, etc
the caseback is the least of your worries
the DW is somewhat recessed and the crystal needs double A/R if ya find one with a properly aligned slide rule