At least with small arms, often the "missing" model numbers are either 1)one-off, special purpose purchases for a single service, or 2)something uninteresting.

At least with small arms, often the "missing" model numbers are either 1)one-off, special purpose purchases for a single service, or 2)something uninteresting. For example, at least 1 issued "pistol" prior to the M9 was a flare gun. The M3 Carbine, ostensibly the "predecessor" to today's M4, was a WW2/Korea night-sight-equipped M1 Carbine. With aircraft, I assume the trend is similar, with the "missing" numbers being trials aircraft or testing designs.