Hi Harry:

Richard Harris' book Discovering Timber-Framed Buildings is based on the craft in England and has a table of various marks on p. 15. Along w/ roman numerals, the mark you refer to there on that barn looks like a number of ways carpenters have used to designate the sequence 1,2,3.

The caption w/ the table in the book reads: "A variety of identification tags attached to numbers 1,2,3."

I have drawn what the book shows below:



The book also shows the same sequence, but w/ whole circles and lines. The semi-circle looks like just another variation. I wonder what they did for the number 4,5 and so on??