A New Engine from Paterson’s Famed Cooke Locomotive Company
When the builder’s plates on the side and front of the locomotive Samuel Dunning posed with are scanned at a very high resolution, it’s possible to discern that the engine was built by the Cooke Locomotive and Machine Company of Paterson, N.J. 
Above is an advertisement which appeared in a book published in 1899, The Locomotive Up To Date by Charles McShane, a 700-page volume covering all aspects of locomotive design and use.

The engine Cooke chose to advertise is nearly identical to the engine Samuel Dunning posed with, right down to the markings of Missouri Pacific Railway.  Except for a cosmetic change to the headlamp, the only apparent difference is the engine number: Samuel Dunning posed with, and presumably delivered, engine No. 765.  It is entirely possible, of course, that he subsequently delivered the very engine photographed for Cooke’s advertisement.
Engine 765 of the Missouri Pacific Railway

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Samuel Dunning’s photographs.