Hello and thanks for posting:
Let me begin by saying that I'm not an expert. I know just about enough to get me into trouble, but seldom enough to get me out of trouble. In this case, I'm pretty sure that your cannon is an iron British naval cannon. The key is the marks on the end of the trunnions. There is lots of information on the subject, becuase these marks are often the primary way that underwater archaeologists identify shipwrecks or what remains of shipwrecks. There's some difference of opinion among the experts regarding the details of the marks as they relate to dates, etc.. It looks like yours dates to 1723, but once again, there is some sort of disagrement over the details regarding dates, espcially of the early 18th century dates. If you do a computer search on British iron naval trunnion marks, you'll find lots of articles with a wide rage of details about the dates of cannons like yours. And, given the British penchant for recording keeping, I wouldn't be surprised to learn that there was a list in some archives that specified where the cannon with number "50745"[?] was originally posted. Just don't let them know you have it; they may want it back! I hope this helps, at least a little. Please keep us posted on what you discover about your "shooting iron." Thanks again.