There was an earlier thread about the red circle on the Mamiya Six lens standard, which several people, including me, think is merely a non-moving red circle. The sync post is behind it as the OP discovered. My guess is that there was a flash attachment or cord that clipped over the red circle and the post, but that none of us have ever seen it. Bear in mind that it would have been in the flashbulb era.
If I were going to try to use the sync connector, I would try to homebrew a cord, perhaps using something like a barrel plug (the kind that is used for DC power supplies) to fit over the post, and some kind of clip to connect the outer terminal of the plug to the camera lens standard.