Yard marker on BO cloth??

I just finished my first DIY screen at 106". I think it looks pretty good, much like michaelddd's. Instead of staples, I used screen spline and a screen frame mill. It holds it just as tight, but may go slower than staples.


Anyway, when I turned it over to check out my work, I saw these small (about 1/8" diameter) gray spots on the fabric that were exactly 36" apart. I only noticed one of those spots on the edge, but that was behind the frame so no one could see it and I didnt worry about it.

Think that my Joannes used these marks for a yard marker?

Anyone else ever heard of this or experienced it?


btw, I have the cloth side showing. The spots are not present on the rubbery side, so I may flip it over.