The original "gunner's hatch" was replaced with a larger retractable entrance door, and room for baggage was provided in the nose section.

By 1963, six Marksman conversions had been carried out for civil customers, the final seventh and eighth being a special purpose version with terrain-following radar and a cargo-dropping hatch for low level air-drops, designed by and delivered to CIA-associated companies.

The official role of these aircraft as stated in the specifications provided to Air America from Intermountain in March 1967 was "Aerial resupply, [and] low level penetration."

[3] Between June and October 1967, the first aircraft, re-registered from N900V to N46598, conducted low level nighttime supply drops to CIA related forces in Laos during the so-called "Secret War".

"[3] In the end, both aircraft were handed over to the 1198th Operational Evaluation and Training (OT&E) Squadron at Norton Air Force Base, San Bernardino, California, a unit known for alleged participation in agent dropping and other clandestine missions in Southeast Asia (Project Heavy Chain).

The Squadron evaluated the two Marksmen, but apparently found no use for them and scrapped both aircraft, which suffered from a chronic Invader issue of nose gear failure.