No. 60 Squadron's Pembrokes were modified versions of No. 81 Squadron RAF's C(PR).1 photo-reconnaissance Pembrokes. Their use as a Cold War surveillance aircraft was highly classified until the late 1990s. Pembrokes of No. 60 Squadron often flew along the air corridors between West Germany and Berlin, established during the 1948–49 Berlin Blockade during which the West mounted a massive year-long airlift of supplies to the beleaguered city. While they were widely used as transport aircraft by the RAF, their true function along that particular route was known only to a few within military and intelligence circles. These aircraft were employed for Operation Hallmark, a sensitive intelligence operation in which the Pembrokes were fitted with high-powered reconnaissance cameras to acquire imagery of Soviet and East German military installations and airfields below the tightly controlled air corridors. These were subsequently analyzed by photo intelligence and imagery experts, who recorded any changes in the Warsaw Pact forces facing the West. Alterations in the order of battle, appearance of new equipment and movement of military units were all items of great interest. At the time of the Soviet invasion of Czechoslovakia in 1968, it was Percival Pembrokes that provided Western analysts with some of the first indications as to where the Soviet ground forces had come from.