The catalogues do not give us any "plate flaws" nor systematical characteristics of these 2 stamps! However, I just happened to spot a rather remarkable characteristic that hardly could have been overlooked!
Out of some 180 40c stamps on coated Wiggins Teape paper, orthogonal watermark, direction of paper vertical - 15 copies did have the same feature of the "4"!
The root block of 5x2 was just under my nose!
The thick "4" is number 3 in the first row!
to be continued ...
P.S.
the former owner of this block wrote "papel holandés"! What made him think so????