While the coin really does nothing to positively date the engine, the engine itself is nothing short of incredible!
I suppose that you could argue that the engine could not have been built before the date on the coin, but that isn't necessarily true either as perhaps the coin represents a repair done to a failed excentric at a later date?!?!
In all likelihood the engine was actually built sometime after 1865, and probably not too long after at that, just by looking at it and its style.
What an incredible find that beauty is, for certain and for sure!!!