Sweden Stamps - Coil Control Back Numbers

Can't find a reference, didn't see one in Facit.

When was the first issue back numbers were used on coil stamps?

Were they used on all coil issues or just some?
Every 10, or different spacing?

Thanks,
Pat

From Jay Smith's site, with examples shown :

There is another collectible aspect of Swedish coil stamps: back numbers. Starting on 16 September 1920, numbers have been printed on the backside (on top of the gum) of Swedish stamps. Prior to the 1970s, the numbers were printed on the rolls of 500 stamps (not the rolls of 100 stamps), presumably for use by postal clerks. A number was printed on the back of every 20th stamp; "2" on the 20th stamp, "4" on the 40th, etc. Examples of these early back-numbered stamps are very scarce. I have seen examples in either red or black from the 1920s to the 1950s, and only black since then, in various type styles. Starting in the 1970s, however, the numbers appear every 10 stamps and are specifically for inventory counting use by postal clerks. Though German coil stamps with back numbers (every 5 or 10 stamps) are avidly collected, this style has not (yet?) caught on for Swedish stamps.

https://www.jaysmith.com/Resource/A...rations.html

Thank you for the info and link. That's what I was looking for.
Pat