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I am reading a book about Spanish California, and the author uses the word "rubric" to mean, I think, "personal mark attesting to a document." I don't have access to the original manuscript documents, so I can't tell if this refers to a signature, an "X," or some other sort of mark. Any thoughts on this usage?

Example from the end of a report (translated by the author from Spanish to English):

This is what we have seen.

Frontier of Santo Domingo, January 15, 1780.

Joseph Velásquez (Rubric)

The Oxford English Dictionary has this definition:

In Spanish- and Portuguese-speaking contexts: a decorative flourish attached to a signature; (also) a mark used in place of a signature. Now chiefly hist.

Notably, it was used in Don Quixote:

It goes very well (quoth Sancho) subsigne it therefore I pray you. It needes no seale (quoth Don-Quixote) but onely my Rubricke [Sp. rúbrica], which is as valible as if it were subscribed; not only for three Asses, but also for three hundred.

However this sense of the word is not in Oxford’s general dictionary. It’s not a definition that the vast majority of people know.

According to this source, a rubric is the flourish or swash under a signature:

A rubric is a flourish embellishing a signature; it's both decorative and a security feature.

[history.stackexchange.com]

I'm not sure how credible that website is as a source, but this meaning seems to be confirmed by other references, for example:

The flourish or rubric in the occidental signatures is defined by a kind of doodle written much faster and without much attention.

Modeling the Lexical Morphology of Western Handwritten Signatures,
PLOS One

Or:

A flourish after a signature; a paraph.

From wordnik.com quoting the Century Dictionary