This is not the Papius specimen or control-mark I’m supposed to be thinking about this very moment, but I don’t want to lose this thought:
The cataloger for MZ 194 describes the reverse control mark as crossed shields and a helmet and thus as attributes of the Salii, armed priests who dance for Mars. This seems plausible but not certain. I’d like to see more crossed shields. I’m also interested if this is the cataloger’s original proposal or if it has been proposed else where. Other possibly Salii related images on the Blog.