Bryan,
Please look at the 1907 Olmstead MAP which incorporates the development plan.
You will note that the South Highway, Railroad Line, Peconic Bay, Cold Spring Pond, Shinnecock Bay and North Highway are all depicted.
The MAP is accurate.
The plan merely identifies/labels the block of parcels to be developed, but the main features, are in fact depicted as they existed in 1907, including the North Highway.
Please look at where the North Highway passes UNDER the Railroad Trestle in the 1907 Map.
It's in the exact same location as it is today.
The North Highway appears in that 1907 map as the North Road appears today, as pictured in your photo.
We know, from contemporaneous advertisements by the Shinnecock Inn, in 1906-07, that the North Highway existed.
I suspect that the trestle will have a more modern date, perhaps in the 30's when many public works projects were completed.
While I"m interested in the dedication plate, I"m also interested in what may be historical markers along the road.
Also, please go to the other NGLA thread posted yesterday.