US Prexie 805 Unusual Color And Paper

Came across this stamp, the 1 1/2 cent Martha Washington prexie Scott # 805 from 1938. What is unusual is that it seems to be printed on dark brown paper. It still has gum (cannot swear it is original gum). The pictures are of the stamp next to a 'normal' #805.
Any thoughts?

Probably badly toned paper.

I agree with eyeonwall. It looks like it has been stored between sheets of somewhat acidic paper and has toned deeply over the years. I would also expect the gum to be OG - no upside in regumming such a stamp. I've seen things similar to this on Prexies in the past. I don't know if they are particularly sensitive to environmental issues (compared to other issues) but they are certainly not immune.

I've seen and have some with that same toning on stamps that were on acidic brown paper envelopes for long duration (many decades) and were probably exposed to humidity and temperature variances, such would be common if stored in a box in an attic or other non-environmentally controlled area.