WT and Brian - thanks for your responses, much appreciated! Your explanation seems plausible enough to me, although I wonder why the bleaching (or whatever happened) affected only the paper and not the ink. I wish I could remember how I came across the stamp in the first place, but it's been many years...
By the way, look for future posts from me. I recently came across an old stamp album that my father-in-law
inherited from his father-in-law, which contains some fairly interesting stamps. Nothing terribly valuable, but it does have a nearly complete set of 1870 National Bank Notes, as well as some other higher-denomination pre-1900 stamps, a complete set of federal duck stamps, at least one interesting Postage Due, and some other interesting items. Unfortunately, most are used and in not very good condition.