If there ever was such a signaling system (which I doubt, be that as it may) it had certainly been superceded by simple text messaging or text broad casting capabilities.
In 1978, it may have been necessary to route comments throught the trading platform. That is no longer the case.
Even if there was a human monitoring the trading in every ticker at every MM (doubtful), there is no need to use a volume signal to pass a message. Just send a text or post to a private message board or whatever.
Given that, I discount all the 911 etc. "volume cues" that people talk about.
If I could afford to buy all of them, I would not need to buy any of them and I sure wouldn't be spending time on the message boards!