I am trying to find out further information about a hand stamp that appears on a 1977 Definitives cover. It’s a simple double circle with the words all in capitals SINGLE LINE BAND in the outer circle and PHOSPHOR in the centre with a line above and below. Please can anyone help please.

The handstamp has been added by a First Day Cover dealer. A lot of Machin stamps were issued as a First Class or overseas rate and with Inflation were only needed as a Second Class stamp. With a few exceptions the single band was reserved for second class stamps.
It would be possible to find similar handstamps on the 3p ultramarine, 3.5p olive grey , 5.5p violet, 6.5p greenish blue, 8p rosine and 10p orange-brown First Day Covers . These values were all issued as two band stamps and then re-issued as a single band ( Second class rate ) with no change in colour.

Hope this helps. Paul

Thank you for your kind help