As we all know, opposing forces gathered intel but it wasn't always correct. The 43rd may have thought it was a newly recruited regiment but that may have not been the case.
Very good point, Anthony. Often, the men had no idea who they were fighting until the battle was over and prisoners identified themselves and their regiment. Unless, of course, the opposing force had some unique identifier. The 3rd Arkansas Regiment, in the Army of Northern Virginia, went up against the vaunted Iron Brigade from time to time (Indiana and Wisconsin regiments, as I recall), and learned to recognize this crack outfit by their non-regulation, black slouch hats. Several letters from members of the 3rd Arkansas mention going toe to toe with "them Black Hat fellers."