I think the number 400 was not came from nowhere,but we can assume there areModel 100,200 & 300, any suggesting.
AJI started off in 1951 as California Airmotive Corp., North Hollywood, Calif., one of the largest dealers in second-hand aircraft. In the 1960s, they modified the Temco TT-1 for civilian/export markets as the A-610 (or T-610) Super Pinto. In 1969, they produced the C Bird, a tandem-rotor test-bed with pilot sitting astride the main frame (this was a former Filper design built and tested under California Airmotive). Also in 1969, they produced the
Turbo Star 402, a turboprop conversion of the Cessna 402 powered by two Allison 250-B17 engines, then a similarly converted Beechcraft Baron as the
Turbo Star Baron circa 1972 and the
Turbo Star Pressurized 414 in 1974 based on the Cessna 414.
California Airmotive was renamed as American Jet Industries Inc., Van Nuys, Calif. in 1973. They sponsored a highly-modified F8F
Bearcat by Darryl Greenamyer called the
"Conquest I", and the
F-104RB (or
RB-104)
"Red Baron", a Lockheed F-104 rebuilt from scrap by the same Darryl Greenamyer to attempt to break the altitude record.
Throughout all those years, there is absolutely no indication that the company used any system of designations for all those modified aircraft. The
Model 400 Turbo-Star, developed from 1974 onwards (later renamed the
Hustler 400) was the first AJI type to carry a model designation, followed by the
Hustler 500 and
Peregrine 600. After that, AJI acquires the Gulfstream rights from Grumman and becomes the Gulfstream American Corp. Although it is possible that the designations 100 to 300 may have existed on paper, it seems more likely that "400" was chosen arbitrarily, possibly to sound higher than all then-existing business prop types.