Hi Don,
Thank you very much for the Photobucket info. I posted a photo. We have not yet removed the sheathing board on the wall adjacent to the post with the marks, so I don't yet know if these are truly marriage marks. However, this post has several other unused peg holes and mortises and I suspect it was previously used in another structure anyway. I SUSPECT that this mark was simply a different form of marriage mark, so that instead of the barn builder having to use umpteen Roman numerals to identify all joints, they simply made a mark like this. I just don't want to guess, because this barn is on an historic property and has tours, etc. If you're interested, you can Google "Dwight-Derby house".
Thanks again!