Dollis Hill ST5(4)d Yellow all over phosphor Block of 12

Great Britain has never knowingly issued stamps with a Yellow phosphor reaction although some Machin issues had a Yellow phosphor reaction due to ink contamination. The Dollis Hill archive contained a few stamps and a few covers where Yellow phosphor had been used. Forster stated in his notes No watermark paper sized with Yellow emitting phosphor good emission but condemned as possibly toxic. The stamps are of Light Reddish Brown appearance and are usually a blotchy print. The distribution of the Yellow phosphor is patchy. Block of 12 with Gum. Ex Edward Klempka Author of the DOLLIS HILL FIND Cat 6000 pounds