From a southerner’s eyes, it looks like an oyster knife.
However, I’m seriously doubting Sherman took the time to have US issued oyster knives made and handed out to his troop’s while occupying Savannah during the holidays. (Sarcasm)
Were there any Union issued mess kits? It looks as though the blade may have been it down to me. There are those much more versed in such things who will hopefully see the piece and be able to id the era of production and usage.