US J29 Postage Due, Possibility?

Yes, that's my understanding as well. Although my stamps do glow slightly orange, I'm still suspicious because I'm, as I said, 99.9% they are double line watermarked. I suppose that I will have to triple check.

I just read, via Google search, that sometimes lake color can glow a bit. Perhaps that is what I have here.

Thanks for that reference. While searching around for it, I came across: https://analyticalphilately.org/doc...ngs_2020.pdf

There is an article "1894 and 1895 Series First Bureau Postage Due Stamps: Part I" and also a "Part II".

I don't want to copy specific text from that document, but the conclusions essentially stated that all vermilion stamps fluoresce, but not all stamps that fluoresce are vermilion. This is specific to the J29-J44 postage dues.

The article in my previous email pretty much reached the same conclusion. It's available as PDF at United States Stamp Society's website if you're a member.