Not to be too picky, and I know the interpretation of the rules have gotten rather loose and sloppy over time, but, at least in the La. UCC which, I believe is uniform, the definition of indorsement is as follows...
(a) "Indorsement" means a signature, other than that of a signer as maker, drawer, or acceptor, that alone or accompanied by other words is made on an instrument for the purpose of (i) negotiating the instrument, (ii) restricting payment of the instrument, or (iii) incurring indorser's liability on the instrument, but regardless of the intent of the signer, a signature and its accompanying words is an indorsement unless the accompanying words, terms of the instrument, place of the signature, or other circumstances unambiguously indicate that the signature was made for a purpose other than indorsement. For the purpose of determining whether a signature is made on an instrument, a paper affixed to the instrument is a part of the instrument.
I would argue that the operative term is signature.
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My opinions should not be taken as legal advice and I do not speak for my employer.