Jimmymac,
What we need here is for Bob Smith to consult with his wife (Ruth Brown), because this is right up her alley. I can't make out the tape on the thumbnail pic, but in the fifth photo it looks to be 18-inches from the cascabel to just past the trunnion, so the gun's got to be longer than 32-inches. That raised crown isn't part of any royal cypher, it's just a bit of advertising used by the British foundry that cast the gun to attract buyers from the merchant market. There aren't any close-up pics of the trunnion faces, but I don't think this gun was made by the Carron Company, the corrosion doesn't look bad enough to have completely eroded the marks.