I think it's pretty much established that they are OS marks, and nothing all that exceptional.
Here's a thread about 'em from way back:
http://www.themodernantiquarian.com/forum.php?thread=6820
K x
It is an OS mark. There's one on the old railway bridge next to my Mum's house. They are weird looking things. There's also a little carving of a sailing ship in full rigging - testament to the sae faring heritage of the Dolgellau region.
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