Ireland 3-D Celtic Cross Issue Of 1941

Could use some help from anyone who knows: Are the rectangles with grids and solid blue bars found in the selvedge of the issue shown related to checking ink color/hue, image or perforation registration, or something else?

Opinion.
Referred to as "Ladders" and or "Pillars"
For strengthening the printing plate, and to prevent selvedge paper being used for forgery production.
The thinner solid blue line, is referred to as the "Jubilee Lines" to reduce pressure on the printing plate.

Thanks rod, that's really terrific information that I never heard before (love this hobby). What reference is it that you show in your post? Looks like a book or catalogue I'd enjoy getting a copy of.
Cheers,
George

Hi George,
I had a feeling someone was about to ask.
I am not sure, I took that lift before I was proficient in image naming.

I will take an educated guess, it would either be.....
"Postage Stamps in the making" by John Easton or
"British Postage Stamp Design" by John Easton

Either Tome is recommended , Mr. Easton is a fabulous journalist / philatelist

If I get the time, I'll flip through my books in the Library.

Postage Stamps in the making.
F (Fred) J Melville
Rewritten and completed by John Easton May 1948
Plate located pp201

Super ! Thanks Rod, your efforts much appreciated !