Oh my goodness. I JUST saw that mark again. In ROSSLYN Chapel. No, I wasn't actually there. I was watching TV earlier and was watching some show where this guy was going on about the different conspiracy theories regarding the Knights Templars. (Yes, I'm a total geek. I love stuff like that.) Anyway, it made me remember the different places that have some rumors and conspiracies surrounding them, like Rennes-le-chateau in France, and Rosslyn in Scotland (which, by the way is supposed to be in line with Rennes in some way...). ANYWAY, I started looking up Rosslyn again, and I found a site that mentioned that this guy was studying the cubes in Rosslyn, which are decorations in the chapel. He noticed the symbols on each of the cubes and he realized they actually signify musical notes. You can read about it here:
http://www.crystalinks.com/rosslynchapel.html
Just scroll down until you see a picture of a man and the heading "Stuart Mitchell". He recreated sound of the music (I believe with actual medieval instruments) and he thinks that if it's played in Rosslyn, it will unlock some kind of secret, based on the vibrations of the notes. (Hey, there are sealed chambers under the chapel. They haven't tried to open them because they're afraid the chapel will collapse. The chambers are supposed to have white sand in them, along with lead vaults. I think they did some kind of imaging to discover that.) The only problem with that is I believe that it said that there are two cubes missing. I don't know if that means they went missing before he started studying them, or after.
Anyway, I thought it sounded interesting, so I followed the link to a YouTube video of the actual musical composition, which I LOVE the sound of. Anyway, the video eventually starts showing the individual cubes. Watch the video, and on one of them, you'll see your mark! It shows up around the 46 second mark. I almost fell out of my chair when I saw it. Here's the link to the video:
https://www.youtube.com/watch?v=cy2Dg-ncWoY
So you have a few possibilities here. First, since St. Clair brought in masons from all over to build Rosslyn, and Rosslyn's in Scotland, the same mason could have worked on your castle, depending on when it was built. Second, this mason could have learned from the mason who made the mark in the castle and used it (but I'm not sure if that was common or not. I thought mason marks were individual. I could be wrong about that, though.) Three, the symbol could stand for a group of masons. Again, not sure how common that would have been, either. Or this symbol could have significance that we know nothing about. Read up on Rosslyn, and you'll discover how unusual its design and decoration is for the time period. It contains Green Men from paganism (and a Green Woman), symbols of Judaism, and symbols of other religions. It could also have something to do with the conspiracies surrounding the Knights Templar. I mean, one of the legends surrounding Rosslyn is that the Holy Grail is there. Another is that it contains (or once contained) the mummified head of Jesus (another said the mummified body).
I don't know if that same mark appears on any of the other cubes, because I haven't finished the video yet. I saw the symbol, backed it up, paused it so I could study it, and then came here to let you know what I had found. Now it could definitely be a mason mark, but this also makes me wonder if that same symbol shows up at any of the other sites that are fabled to have Knights Templar connections. Also makes me wonder if the castle is purported to have those connections.
Either way, let your imagination go wild with that story!