Take a look at this "study" done in England. I sent the author an e-mail a few minutes ago to see if he knows any more.
http://www.dbrg.org.uk/Marks/Cmarks.pdfI'm beginning to think that these old builders had a raceknife in their pocket (A race knife is a cutting knife used in shipbuilding, having a blade that is hooked at the point, and is used for marking outlines, on boards or metals, as by a pattern.) and they simply used it to make just another mark instead of using 100 or more Roman numerals to mark joints. It looks as if I and many of you have been going a bit crazy trying to find a better or different answer. This one's good enough for me! Off to the next mystery,
Harry
