Tony - The first DD9G S/N was the Buster Peterson "Quad-Trac", produced in 1965. Note that the "Service Reporter" magazine that reported the release of the "Quad-Trac" was Issue Four of 1965 (Sept-Dec).
Thus, the Peterson "Quad-Trac" was built from late 1965 to 1968 - when Cat started producing the DD9G with 90J/91J S/No's. It appears that the Peterson "Quad-Tracs" used regular production D9G S/No's.
According to my book, there were 51 of the DD9G's built, but only 7 of the DD9H's. Eric Orlemann claims there were 10 Quad-Trac D9G's sold. I can't tell you who bought the first DD9G.
It looks like a lot of the DD9G's and Quad-Tracs were separated and used as individual tractors - and probably sold off as individual tractors as well.
There's some good pics on "QuAD's" website, of a DD9G working in the Maastricht ENCI Quarry in the early 1970's. According to "QuAD", as far as he knows, this was the only DD9G sold outside the U.S., into Europe.
According to Deas Plant, Bell Bros of Western Australia purchased one DD9G and it was used at Alcoa in Pinjarra. I have no knowledge of that tractor.
http://www.bouwmachinesvantoen.nl/bedrijvenpagina/Curfs/curfs.htm