Above is four signals which show different trading/brokerage firms to indicate their activity. Notice how simple they are just as 'modern' day signals were as well.
The four months listed, only July is the same. Sept and Dec got much more efficient and easier to signal as the market needed arb speed 20-25 years later.
I have to admit that signals were far more advanced at an earlier period than I imagined before stumbling upon this article.