Re #27281 Accept spec nums other than 4 or 5 for SANS Offset

Accept spectrum numbers other than 4 or 5. LOQ, for example, uses a
spectrum number around 17000 for their offset, which internally is
mapped to monitor 4. The exception is ZOOM which also can use monitor 5,
however it is (currently) the only instrument using spec 5 so a simple
check will suffice