I was of the understanding that these were very rare. There were plenty of them all empty mind ye at a loch I was fishing. I also thought they were to be found at running water. This was not the case.
you get them on lochs too. I saw them many years ago on Rescobie loch (a large stockie bashing dub)
Loch Fitty in Fife had them years ago too
Thank you for your info Breac Uaig. Thought these ones might be common as there were dozens of them in the Loch.