Blue Thread On Front, Not So Much On The Back

I found what I believe is a R15d. The front has a plethora of large blue threads that are easily visible to the eye. The back however, only has two small barely discernable threads. The other silk papers varieties that I have found have had the visible threads on the back, not the front. I'm new at this so maybe this is more common than I think. Or, maybe this isn't a R15d after all. What say you?

from a scan it is hard to tell but if the threads on the front are under or in the mix of the paper (use a good glass). if they are on the surface only then it is not. the blue drug paper wrappers of that era shed and these can often be mistaken for silk. the back spots should also be also in the mix. they should be blue and not black or brown.

Then that explains it. Close up looks at the front show that all of the "threads" appear to be on the surface.

a not uncommon happining......

Check out Revenuecollector's wonderful website. https://revenue-collector.com/pages/3552.html A quick bath in some distilled water should loosen the fibers if they re not attached to the paper. You can use a small soft paint brush to wisk away any material the water may have loosened. Typically the blue drug wrapper fibers would be stuck to the back of the stamp as it would have been adhered to the wrapper by the adhesive. If the fibers remain, I would conclude that it is a silk variety. Note, that most often the blue, and sometimes red, fibers are more prominent on the reverse of stamp. In any case Revenuecollectors site is an amazing resource for collectors of such material.

Edit to clarify posted link. The value of the stamp linked is due to the double transfer, not being silk variety, it has a very small catalog value as silk.

Thanks for the responses everyone.