So What's Going On With This Bugs Bunny Stamp? Scott 3137A

Hey All,
I'm come across this cover recently that has one of the Bugs Bunny stamp, Scott 3137a. It's postally used, but what kind of oddity or error is this or is this some sort of trickery? You can barely see the date in the lower left corner and there's some white space at the upper left where the face value and USA was/is located. I'm really at a loss as to how this occurred. Chemical modification? Any ideas?
Will

Could this have been done with a white plastic eraser, perhaps Staedtler Mars-Plastic?

When the United States Harry Houdini commemorative stamp was new, Rob Haeseler put the eraser to it and made Houdini disappear. He wrote it up in Linn's.

It's definitely possible it was erased, but erased before or after cancellation? I'll have find another cover with this stamp and give it a try.

ed. I just found out Rob Haeseler passed away two years ago.

It looks to me like it was airbrushed with some kind of white paint, which the subsequent cancel ink had a hard time sticking to.

ed:
I just found out Rob Haeseler passed away two years ago two minutes ago.