I personally have never seen a trunnion cap like that, but maybe others have.... It might be earlier than civil war, as I personally am not aware of any CW ones that look like that with the appendages short like that. There was a lot of strain on a trunnion, taking the brunt of the discharge of the cannon.
Some lathes and heavy equiptment had trunions , or anything that needed a 'bearing' surface might have them. Welcome to the forums !! David.