Strange Numbering On Spanish Stamp Series "Cadet"

I'm not an expert on the series and the issue I'm going to present regarding the Spanish issue of Alfonso XIII "Cadet," but I am familiar with these stamps since I have a modest catalog of 15c impressions. That said, one characteristic of the issue is that the back of each stamp has a serial number that can vary in color (from blue to green) from one of the six different printings that were made. The curious thing about the stamp I'm going to show you lies, firstly, in its paper, which is slightly greenish. But putting that aside, which might be a nice minor detail, the important thing is that the numbering on its back doesn't match the font of the rest of the issue. The last five digits are much thicker, while the first one, the "8," is in the same thin font as the rest of my stamps. As you can see, even the color seems bluer than the other digits, so it appears to have been added independently. It's really puzzled me, since I haven't seen similar numbers in my own collection or in internet searches.
A good article about the issue http://www.filateliaincidental.net/...adete001.htm