I just stumbled across this four-year-old thread in the process of researching the issue of whether the tin whistle would have been played by Civil War soldiers in camp, etc. First, I would ditto Elaine Masciale's comments. I play tin whistle in a CW string band, but I can't play a transverse instrument like a fife or flute. I'm vitally interested to find some scholarly archival evidence on this.

I'll offer several points to add here. By all means, I'm interested in any additional info from anyone, especially if it contradicts or clarifies what little I have been able to find. One, the tin whistle, also popularly referred to as the penny whistle, is not made of tin. It was rolled sheet metal -- I suppose that would be iron, as steel was still quite expensive -- covered/plated with tin to prevent rusting. There also seems to be no real evidence that it sold for a penny. Perhaps "penny" is more a reference to its being inexpensive.

I've found no evidence to its being used in place of the fife in military field music. The whistle, unlike the loud, shrill fife, just doesn't have the volume for that job. So, while the two are similar in appearance, there is a great difference in volume. The tin whistle, if indeed present, would have been a welcome sound around a campfire or inside, but not loud enough for field use, particularly in the din of battle. The converse would apply to the fife.

I do know that fifes were predominantly pitched in C or B flat, before B flat became the "standard/default" key for military fifes. I do not know just when that evolution came about. So, that this whistle is in the key of C is interesting. Was its heavier weight perhaps an attempt to create a louder whistle that could suffice for the volume of the fife? Is the patriotic design on the whistle of official military use, or was it just for general patriotic sentiment, to appear to be military (perhaps for a boy's toy)?

A Civil War flute and fife musician whose opinion I would give great to expressed to me this weekend that metal flageolets and flutes would not have been present during the Civil War. He also remarked that he has never found visual appearance of what we would call a tin whistle in a Civil War era photo or drawing, nor has he found any written evidence of the instrument in diaries, letters, etc. He is a very accomplished tin whistle player, but he doesn't use the instrument in his CW music.
Soooo, this particular find, as in so many historical cases, creates more questions than it answers. It can't yet be considered an exception that disproves the rule. There is much more research to do.

BTW, the question as to the presence of the harmonica is in more or less the same state.

Howe in his "School for the Flageolet", 1846 shows fingerings for the "English flageolet, which I had understood to be the tin whistle.

Just found complete copy of "Howe's" online. It refers to the English flageolet as having a hole for the small finger and another "behind" for the thumb. It refers to the French flageolet as having two holes "behind" for the thumbs. Neither would be the six-holes-all-on-the-front configuration with which we're familiar today. That didn't take me in the direction that I hoped it would. "We're drifting too far from the shore."