1st class gold Security Machin forgery.
One of several types which have been produced.
The iridescent printing is not visible under the 'phosphor', so only MAIL is visible in the year position.
The stamp has appalling detail on crown, with text barely visible
The 'phosphor' bands of course are not phosphor, but do react yellow under UV.
Two examples, on paper, one with packet postmark, and one uncancelled.