At the moment, having not found any indication about the USN usage of the HHD2 neither on the sea nor on the land, the other most likely candidate to the 3rd open position is
Giorgio Michetti
(Francavilla al Mare, 29 May 1888 – Roma, 4 February 1966)
He served, as Silvio Scaroni, in the 76a Squadriglia and was a close friend of Scaroni himself.
He was credited 5 officially recognized victories in WW1 and used to fly an HD1, that was enough to be called an Ace...
He marked his planes with a white seahorse in a black circle.
Of course he had the same painting schemas of Scaroni (except for the personal marking, of course).
But I found this profile on the book (6):
In the same book there is this photo:
taken in spring of 1918 of the full Squadriglia Line up in their last base (Casoni, near Castelfranco Veneto - Treviso)
Where you can see the seahorse plane
but also the Scaroni plane
Painted dark and with the personal marking inverted tones...
So, not to make 2 identical planes I prefer to use this alternative livery.
Scaroni's HD1 7517 was the last plane he used and was adopted by him in late June/early July 1918.
