I think the unusual muzzle will be the key to ID but if no one here recognizes
that style
It sounds like you've been looking for similar pieces somewhere, which I don't think you will ever find. Many of this general type of saluting gun was made as a one-off, so this will be the only one like it, in my opinion anyway.
Here's what may have been a typical scenario. Someone, a vet's unit or whatever, wanted a parade/salute cannon and went to the nearest foundry. That foundry had never cast a cannon before so they sent the vet's post rep back home to make a sketch of what he wanted. When he came back with it, they sent him to a local pattern-maker. He came back three weeks later with a wood pattern. The foundry worked that one-off into their production schedule and told him to come back in about a month. He did, and picked up the casting in his wagon, and took it to the local machinist for boring and finishing. When he got it back from the machinist, the appointed or volunteer vets at his post got busy and cobbled up a carriage for it. Sound like a possibility?