Whilst sorting out some Danzig stamps in my collection and using the internet to research an item I have stumbled across a term I had not heard befor (after nearly 60 years of collecting!) - Burelage - the wikipedia definition is given below -

"Burelage (French: burelage), also burelé, is a French term referring to an intricate network of fine lines, dots or other designs printed over or as the background of some postage or revenue stamps to prevent counterfeiting. In English the word is sometimes spelled with an accent on the first "e" as burélage, although the accent does not appear in the French spelling and its origin is unclear. Burelage most commonly appears as a form of underprinting.

Early uses of burelage on postage stamps include the first issue of the stamps of Denmark from 1851, and stamps issued by the City of Hanover beginning in 1855. Stamp varieties may be distinguished in catalogs based on the presence or absence of burelage as well as variations in the burelage itself, such as the size of network, orientation on the stamp, color, or method of printing."

Burelage is usually unobtrusive which is why I have probably never noticed it so I went on the hunt and sure enough I had some exampleswhich I was about to add to circuit book pager to sell off! (Now ensconsed in the collection of course). From what I can find without a specialised Danzig catelogue, Michel numbers 41 to 46 have a line type of burelage that can be oriented either up or down depending on how the paper was fed into the press. Examples are shown below.

Similar issues from Germany apparently also carry this feature so the hunt is on!

Danzig Issues Mickel 73 to 88 have a rosette pattern Burelage as can be seen below as a network of fine lines extending to the perforations