Penstock Valves were originally the gate style valves used to close off the flow of a penstock. A penstock is a sluice, channel or pipe used to direct fluid to a desired point. e.g. A penstock feeds a turbine generator in hydro-electricity generation by piping it from the base of the dam. An isolation valve used in this application would be called a Penstock Valve.
It appears also that the design of the particular gate valve has become known as a Penstock Valve. These valves are also known as parallel-slide gate valves or knife-gate valves, depending on their particular configuration and usage.