As many of you know, I only collect Newfoundland. Here are my ignorant thoughts:
1. I am dubious of it being authentically used and agree with SPQR. I would however invest a small amount and send it to VGG to be certain.
2. The stamp itself looks too good, so clean and large borders are rare. So i'd make sure the UV analysis is done. And check the paper. My guess is that it would be Unitrade #20 in 1861 (the later or Third Pence issue, which was engraved without watermark, though some had a Stacey Wise two line watermark. I rule out the other two below). According to John Walsh's NSSC : there were 70,000 of these stamps made, about 1,531 were watermarked).
3. The bisect. Per Walsh, there are only 4 known bisects on covers. It was unusual to say the least. His source is Pratt (more on him later). The fact that this stamp is on a cut corner raises doubts to me--why would someone cut the corner off a cover? Collectors then knew, the destination matter, as shown by the great example above. By the way, the earlier issues (#6 in 1857 has only been seen on 1 cover and only 20 stamps are known to exist, so we can rule this one out. #13 was orange in color, so not that one).
4. If the cancel is correct, in this year, there would be other stamps available to make 3cents, so another niggling doubt in my view.
5. Pratt : This book was seminal and while it has some errors, it is definitive. He did a census of all the Pence covers and published it in 1981. Some covers but few have come to light since then. On page 154, Pratt only lists 3 uses of #20A, in use as Bisect and all were to St Johns in 1863. John Walsh would have found one additional example that came to light (I don't have the details). But one can assume that bisects of this stamp were used TO and not from St Johns prior to the date of the cancel.
6. The cancel. I can't be sure, but it looks false to me (the circle, the font, the number). I can't say for sure but there are many exemplars of this post office at that time but I don't have them handy. VGG would.
I thank you for sharing. I am not sure but if I were in your shoes, I would send it to VGG just in case. My guess is that it is a real #20 (costs like $20-30) and placed falsely on a corner with false cancel.
Here is an example of a #20 in use from 1865 from my collection:
http://newfoundlandcovers.org/cover...865/#Details