although I can't see a picture of a seal in the book, Wedgwood certainly did produce one, and bluey greens were one of their colourways. In Susan Tobin's book the seal has the code No. RSW 237 (SG237).
Just thought it worth mentioning - as if this were a Wedgwood example then the possibility of finding a replacement might be easier than if it were 'of no known make'. Of course, you may already know that this seal has nothing to do with Wedgwood.
Just a thought.