I accept what Frank says, as a cautionary note.
Is there any evidence of "copies" of script "P. Ysart" signatures?
If the signature has been added by somebody other than Paul, then whoever did it was incredibly good with the basic features and balance of spacing etc. All the features of this one tie in very well with others I have seen - even though there are minor variations in parts, something which seems consistent across the other sigs I have seen.
In connection with his dealings with Paul Jokelson in the US, Paul Ysart often visited PCA conventions, where collectors discussed with him some of the items they had. Although I can't prove it, I suspect that Paul may well have signed a number of pieces on request in America - it should not have been too difficult to get hold of an engraving tool.