You cannot use only part of a field nor make up your own switches, so with a bit of lateral thinking the following construction should work
{ SET A { MERGEFIELD r1_srl \@"AM/PM" } }{ SET B { MERGEFIELD r1_rsu \@"AM/PM" } }{ MERGEFIELD r1_srl \@"h:mm" }{ IF A = "AM" "A" "P" } - { MERGEFIELD r1_rsu \@"h:mm" }{ IF B = "AM" "A" "P" }
You might find it simpler to use
http://www.gmayor.com/export_field.htm to convert the text string above to working fields.