 Three Wallace Is ("K4346"-"K4348") were later converted to Mk II standards, including fitting the closed canopy.The original PV-6 prototype, registered "G-ACBR" (and also known as the Houston-Wallace) along with Westland PV-3 "G-ACAZ", was part of the Houston Everest Expedition, named after Lucy, Lady Houston the patron, an attempt to fly over Mount Everest.Both aircraft received modifications that included fitting heating and oxygen equipment, fully enclosing the rear cockpits and using highly supercharged Bristol Pegasus IS 3 engines.