I hope it's an "elevation screw to adjust a cannon for distance".
But it looks more likely to be a relatively modern wall anchor bolt & mount plate.
I'm basing this on what looks to be a modern hex bolt under the flat metal plate.
Tony_in-SC is correct. As he said... "An elevator screw [for a cannon barrel] would have a very coarse "acme" type thread." If you care enough to want verification of what I'm telling you, do a google-search for images of civil war (and earlier) cannon carriages.