Archaeo: "Are there instances of metaphor depiction?"
Lee has given one example of the pintailed duck used as an ideogram for son.
This may have been for purely phonetic reasons but it seems likely there was also a symbolic meaning.
"As a phonogram, the sign was used to write the word sA, "son," and this explains the frequency of its appearance in the Egyptian inscriptions that were always careful to identify a person by patronymics and matronymics: "The N son of N son of N" and so on, going back sometimes far enough to cite the long genealogies recounted to Herodotus by his Egyptian interlocutress. Horapollon himself (and the late tradition of mixed Greco-Egyptian culture, from which he drew) saw a causal connection rather than the purely phonetic one that relates duck to "son": "When they want to write son, they depict a goose. Because this is the animal that shows the most affection for its offspring, and if someone chases one in order to capture it with its young, the father and mother offer themselves spontaneously to the hunters so that the young be saved. And it is for this reason that the Egyptians have thought it just to worship this animal."" (Betro 1996: 109)
If the 'forearm with palm down' sign had a similar usage, then what is its symbolic meaning - any suggestions?
CT: ".... this suggests the cubit was originally derived from the length of an idealized forearm....."
Archaeo: "I cannot help but comment that the above, as excerpted, seems an assumption."
Its a question of what is more or less likely.
Archaeo: "Could it be the forearm is the ideal depiction of the length of a cubit?"
Based on the hieroglyphic evidence, its likely the ~524mm cubit was derived from an idealized forearm as the 'forearm with palm down' sign represented the cubit. Similarly, the main subdivisions were represented by the 'hand without thumb' sign for 'palm' (1/7 of a cubit), and the 'finger' sign for 'finger' (1/28 of a cubit).
One of the earliest occurrences of the word cubit (mH), is found in the early Dyn 4 tomb of Metjen, where its simply written with the 'forearm with palm down' hieroglyph (D42). This sign can easily be confused with D41 or D36 as only slight differences distinguish them, but the main concern here is that, at least from as early as Dyn 4, it represented the cubit.
CT