This locomotive is a mostly new replica of a World War I Davenport locomotive, used by US forces in France. The frame, drivers and cylinders are original; everything from the wheels up is new. It has air brakes, which were not used on the originals. Owned by someone in California, I understand.
This is a recreation of Davenport 2967 of 1918 which, along with sister loco 2968, was turned into a 2-6-0 tender loco by Byron Hiatt of Creston, Iowa. An interesting detail of the rebuilt loco is that it has Ffestiniog-designed 'chopper' couplings.
Addendum: Discussion of the identity of this loco on the British NG Enthusiasts site seem to indicate that it was actually US Army No 5240, Davenport works number 1695, and Byron Railroad No. 2967. US Army 5245 was Davenport's number 1700, and Byron Railroad No. 2968. The man behind the project is apparently Peter Nott, a Ffestiniog Railway supporter who has spent long periods in UK over the past few years. He passed out as a steam loco driver on the FR some years ago.