Just the numbers. The real Niagaras were all alike, except for the first one, #6000,
class S1a. The rest (through 6024) were class S1b. The 6000 had a slightly different
trailing truck, smoke deflectors and different class of PT tender - and a multitude of
other little first-one-of-the-class differences, I'm sure. It tested on the NYC for quite
a while (a year?) before the rest were ordered.
Now, the 26th Niagara, #5500, class S2a, was VERY different: it had poppet valves with dual steam admission pipes (visible); air pumps mounted on each side instead of on the
pilot, as the poppet valve gear took up that space; no eccentric crank or rod due to the
poppet valves (your car has poppet valves); a different number series - of 1.