It would be helpful to see it more this size:
This is a #125 scan from the Philatelic Foundation site. Note the very crisp lines of shading. Note in particular that the smoke above the engine is made up of distinct crosshatched lines. Compare your stamp. Further, the paper for the reissues is different from the 1869s, harder/crisper and white rather than off-white. If you have other 1869s, compare with those.
You might also trying looking at the front and back under a very strong light held at a very flat angle to the paper. Some grills are very weakly impressed, but a trace can be found usually.