It is actually a German 155mm field gun or howitzer and was manufactured in the Krupps factory near Munich for use in WWl. There were formerly two of them on Cumbrae and they were mounted as a pair near the current children’s play park in West Bay. There are various stories of how they came to be on the island. Some say that they were provided to train the local infantry volunteers, other that they were for the defence of the island during WWll and it is even speculated that they were brought back as a prize from the first world war. It is likely that the actual reason for them arriving on Cumbrae will never be discovered. How the last of these pieces of armlament came to be buried in a wall is also something of a mystery. It is thought that during the time that railings in the front of houses were being removed to be melted down to help the war effort that some enterprising locals buried the gun for safekeeping. The fate of the second one is also subject to speculation that it was taken out and pushed over the site somewher in the Clyde.
You can see this one if you head out past the playpark in West Bay and go seaward to the boundary wall just before the new toilet block. Take a look at it but please do not climb on it as it is getting fragile.