Fingers are cupped to form a C
C is first letter of company. Also this was formerly used for Citigroup before they left and previously to that Credit Suisse.
The ability to recognize all market participants was what made open outcry trading transparent and from this, signals were developed for the larger brokerages and for some large local traders. In cases where a more universal signal was never developed, their hand signal in the pit would simply be their three digit firm number