Frankly, I think it could be a marker or similar from any industrial use of the nineteenth or early twentieth century: canal, dock, factory, mine, etc. Anything where somebody might be told to stop at or beyond, or not to pass the A marker; there might well be ones marked B, C, etc. (And that is just one possibility.) It was quite possibly a fairly standard design put out by an ironworks and, unless there is some owners or helpful manufacturers mark, I doubt if you will ever know. Even if you did find a photo of one in use, it could just be one of a number of uses. Sorry to be so dampening.