The order to convert 72 Priests into carriers by the commencement of Operation Totalize on August 9 was given on July 31 by Brigadier C. M. Grant, the Deputy Director of Mechanical Engineering at Headquarters. Ultimately, 78 would be converted prior to the first engagement, in spite of the fact that the operation's start date had been advanced to August 7—an impressive feat, as the full engine overhaul included was generally a seven-day operation. The drivers for the new vehicles were swiftly and secretly recruited from the Armoured Corps reinforcements, artillery units, and the Elgin Regiment, and were rushed into service with almost no training, first seeing action during the attack on Falaise on the night of August 7–8, 1944.