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"

Others
