Hi,
An explanation of the term, "signed all over":
This usually means that the piece is its own signature. If you prefer, it is a shorthand way of saying that it could not be by anyone else, since everything about it suggests that it could only be produced by the person/manufacturer being referred to .... (in this case Leerdam - in the opinion of the seller in question).
It does not mean that there is a physical signature that you can read.
Nigel