This was a new way of automatic letter sorting tested in the 1960s. Phosphor is a type of coating that was put on the stamps and could be recognised by machines. During the experimental period in the mid-1960s, commemorative stamps were issued two formats: with phosphor bands (phos) and without (non-phos). These days all stamps have phosphor coating (if it is missing it would count as an error). To see the phosphor you need to hold up any recent Royal Mail stamp to the light. The phosphor appears as dull strips on the stamp.