For your reference:
This is Scott 26. This should almost be a slam dunk for identification, just by the color:
erosion around the perfs and all.
I was just watermarking before even sorting a few WA stamps by design. And I found this with a sheet watermark:
Going to the Stanley Gibbons catalog, they do indeed note a sheet watermark T H SAUNDERS 1860 for this. Here's the N and the D (reversed) showing on this stamp. There are other early stamps with that same style of sheet watermark.
Also note that there's a carmine rose listed on unwatermarked paper, just for more confusion.