Dave Price e-mailed me with a question. "What is Phosphor Under Ink (PUI) and how can I find out if I have these variations?"
Phosphor under ink :
Certain stamps printed on FCP have a milky appearance, this is caused by the phosphor being printed first, then the stamp image printed over the top. The best way to test for these variations is is to hold the stamp up to the light, the bands will appear milky and lighter than the none phosphor area.
Two values that come to mind are the 4½p Pale steel blue (Oct 1974) from Booklet Panes DP23C, 6P pale green from sheets, cyl 8 (Oct 1973) and 6p (Oct 1977) cyl 4 (printed on the Halley Press).
There are others, 11p, 12½p to name another two , but without checking a current catalogue I do not have the source or printing details to hand.
More information on Phosphors can be obtained on my website here and here