As you say, none of the standard production colours or shapes come anywhere near this, but.........
Susan Tobin uses the expression............"on Crystal produced during the early King's Lynn years......" - which assume means the late '60's - so the word crystal is not necessarily inappropriate.
I can't find mention of a label that matches this one exactly, but elements of the wording agree with other labels which were curved and oval in shape, and we've no reason to think this is a forgery - 'England is in the correct position to match similar labels.
This blotch-like effect with the colouring does come near to some of RSW's early experimental pieces - although obviously with one off's there aren't going to be real matches with other pieces.
What is the wear, if any, on this vase? If the base is very unmarked I'd be a little curious.
Sorry this is not conclusive, but you could be in with a chance - let's see what the others think