Anyone who drives down Kuhl Road will see this bridge. It's right on the back corner of Heron Glen golf course. Where Kuhl Rd takes an abrupt right hand turn when it hits the Neshanic River, this bridge crosses over. It's in an ideal position ... but it was never needed.
Kuhl Road has been around since middle 19th Century or longer. The north-south highway just to the West now called Route 31 has also been around that long. Before the Heron Glen golf course was opened in 2002, the property was known as the Kuster Farm, and it stretched from Route 31 to Kuhl Road and perhaps beyond.
I can find the old road that the bridge is on, on a Army Topo map from the 1950's: http://elibrary.rutgers.edu/quadpage/viewmaps/drgfiles/04007... (top left corner, go across the top edge just a bit to where it says "Three Bridges 3.2 Mi". Now go straight down a little, to about 44.80.5. See the word "Neshanic"? That's Kuhl Road and this bridge is just SE of the "c" in "Neshanic". Zoom in on the map and you can see where the old road met up with Rt 31, at what is now called Hampton Corner Road, where the 3rd branch of the Neshanic crosses Et 31.)
That map was updated in 1970 and again in 1977, but by the time that Hunterdon County did it's bridge survey in 1992 (http://www.state.nj.us/transportation/works/environment/pdf/Historic_BR_Hunterdon.pdf) no mention of this bridge is made AT ALL. And that's a full decade before the golf course was opened, so in all likelihood the land was still Kuster Farm. And yet ... no bridge. The Kuhl Road bridge mentioned, #10XXQ48, is the 1936 Pratt Pony.
The style of the thing bugs me too. It's a Howe, a very basic, simple Howe truss. Except nobody was making Howe trusses in the 3rd decade of the 20th century; even if I could accept that some not-so-bright builder "put the diagonals on backwards", nobody was building Pratt through trusses at that point either. Hunterdon County has dozens of trusses, probably 100 or more, and we've got "new" examples from 1914 right up to 1940: rivets are on their way out by the mid 30s, as can be seen on the Kuhl Road pony just down the road. But those bridges used gusset plates, and they used different weight H beams for different parts of the bridge. And almost all of them are Warrens. This one is made of nothing but 4" H beam, top to bottom. It also shows signs of once being painted white, like the other bridges in the county once were. So I'm thinking it probably was a county bridge, and that Hampton Corner Road once meandered its way across the hilly field to connect to Kuhl Road across the Neshanic. But it probably wasn't a paved road, so it fell into disuse?
That same road can be seen here, in this 1914 map. http://mapmaker.rutgers.edu/HUNTERDON_COUNTY/Hun_Som_1914.jp...
Center the image top to bottom, then look at the left third. Find the big letters "HUNTERDON" then south of them "Flemington", then just below that "Copper Hill". Just to the right of that, under the "S" in "PENNSYLVANIA" is our missing road ... and this bridge is at #67.
Farmers are world famous for not spending money if they don't have to. With access directly to Route 31 since the beginning, and access directly to Kuhl Road which wraps around their fields (also since the beginning), what would be the benefit of the Kuster family building their very own bridge? Or any other farmer? None that I can see.
So, how old is this thing, and who made it, when, and maybe even why? And why is there no record of it? I really can't answer any of those questions. But I can easily see that it was foolishly made, and bent under load probably soon after. That's why I thought it might be a homemade bridge at first.