Perhaps Herodotus for it is said that he compaired the two larger.

Speaking of Herodotus.

In former threads there was some misapprehension as to what constitutes a cubit. Most metrological findings state that the arm'slength, from finger tip to elbow, applies to the short or common cubit of approximately 18 inches. The Royal on the other hand is somewhat longer.

Here is Jomard's take on the arm'slength, illustrated as the equivalent of 24 "doigts", presumably 4 "doigts" less than the Royal cubit. It is therefore obvious that as early as the 1800's this distinction between cubits was made known.

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Herodotus on the other hand, some 2300 years prior, said of the two mainstream cubits ... one is "three finger-breadths" less than the other and in biblical times we have “a cubit of a cubit and an hands-breadth”

I doubt whether anyone still believes in a royal cubit arm'slength, unless it applies to a larger than life specimen, Pharaoh most certainly wouldn't have qualified. As for 4 digits verses three finger-breadths your guess is as good as mine.

The French mathematician Edme-Francois Jomard accompanied the Napoleon expedition to Egypt.