Hi Jon,
I don't have my copy yet so I can't see how that illustration turned out
Duncan Head suggested that Xenophon's “golden eagle” was the winged Faravahar symbol from Achaemenid art. I think its more likely that the “golden eagle” looked something like that blue faience tile from Persepolis, or a gold plaque of a hawk from the Oxus Treasure. As you say, this is supported by the head of a bird on the Alexander Mosaic standard. But the other theory is possible, so when I saw Igor's sketch with the Faravahar symbol I didn't object.
For your interest, I've attached a scan of some of those early sketches of the Alexander Moasic showing the bird's head.
[img] Two 19th century sketches of the Alexander Mosaic[/img]
Sean