ClipArt ETC
"If only one indicator is to be used for both ends of the cylinder, it may be connected by side pipes and a three way (pipe valve). By this method both diagrams are taken on the same card and with the loss but one revolution." —Derr, 1911

Three Way Pipe Valve for Attaching One Steam Engine Indicator

"If only one indicator is to be used for both ends of the cylinder, it may be connected by side pipes…