Danny, other than as half the turn and slip indicator itself, was the other "ball" simply an added part to another instrument? It occurs that for whatever reason the US Govt might have required required that a "ball" be attached to a particular instrument (artificial horizon, directional giro?) to aid the scan and ensure an in trim aircraft. It was the most basic of devices and would add mere cents to the cost, especially in the prodigious numbers being dealt with. Just a thought...