That is a very yummy Cinderella!
It's not a test stamp, it's the joining tab, that maintains
the coil from unwrapping prior to installation.
Or, if you will, the "leader"
A very tasty piece of paper.
Allow me to correct myself,
(memory chip failure
There seems to be a new word creeping into philately,
This is a US "Dummy" stamp
and as you say was probably a stamp from a roll used as a demonstration.
I had trouble recalling the last time I saw these,
and I had them under "affixer" as that was a common
reference text string, downunder at least.
There is a web site out there for US under "dummy" stamps
Here is an article from the NZ stamp club "The Captain Coqk"