 Complete the following phrase:  A heavily modified pair of OH-6As were utilized by the CIA via Air America for a covert wire-tapping mission in 1972. The aircraft, dubbed 500P penetrator by Hughes, began as an ARPA project, codenamed Mainstreet, in 1968. Development included test and training flights in Culver City, California Hughes Airport and at Area 51 in 1971. In order to reduce their acoustic signature, the helicopters N351X and N352X received a four-blade scissors style tail rotor later incorporated into the Hughes-designed AH-64 Apache, a fifth rotor blade and reshaped rotor tips, a modified exhaust system and various performance-boosts. Deployed to a secret base in southern Laos PS-44 in June 1972, one of the helicopters was heavily damaged during a training mission late in the summer. The remaining helicopter deployed a wiretap near Vinh, Vietnam on the night of 56 December 1972, which provided the United States with useful information during the 