There are also the 56 Aubrey holes around Stonehenge. These are suspected by some to mark the lunar eclipse cycle. 56 = 19 + 19 + 18, approximating the 18.61 cycle 3 times.
The Wikipedia article indicates that this particular "hole" theory does not work.
Exactly why I used the word "some". I remember watching Dawkins' show back then and being excited that it was possibly being used as a calculator back then.