Just to drift the thread a tad and add some late 1950s, early 1960s history, the On Mark Marksman was sold as the ultimate corporate transport of the time to the US business market.
Whilst earlier convesrsions of WW2 and late 1940s military airframes had been just that, so that the aircraft were basically bombers/attack aircraft with seats and some soundproofing, On Mark, which held an A-26 spare parts manufacturing licence from Douglas and knew the military version inside out, decided that the spare airframes coming up for disposal had some potential to provide fast, exclusive, personal transport.
They decided to totally rework the airframe. The first iteration was the Marketeer which was unpressurised. The ultimate was the Marksman which was pressurised and was basically, when On Mark had finished with it, a new aircraft.
Both types looked like Invaders, stripped of guns and warpaint, both sounded like Invaders and started life as such but the Marksman was so different it gained a supplementary type certificate.
Similarly today the BBJ has a seperate certificate as it is an amalgam of both the 737-700 and 737-800 and the A319CJ is also certificated differently to the A319. In contrast the earlier Boeing, Douglas, Airbus, BAC, DH etc. types used as executive jets were all certificated as airliners.