The M151 (MUTT – Military Unit Tactical Truck) was produced from 1961-1984 as a replacement for the M38. Designed by Ford and built by Ford, Willys, Kaiser, Jeep, AM General Corporation, and GM, some critics believe that it was unsafe and prone to rollovers, partly due to its use of independent front and rear trailing arms rather than live axles.
In 1964, Ford started making the M151A1; it was an attempt to fix the M151's problematic handling. Besides its new fender-mounted turn signals, it had a stiffer rear suspension, firmer mountings, and an extra rubber bump stop to help eliminate rebound. Yet the rear wheels could still swing down and under when unloaded, because the original suspension arms remained the same. The A2 replaced the A1 in 1969.
Tannon Weber wrote about an M151A1:
Among the vehicles at the Titan Missile Museum (south of Tucson, Arizona) is an AM General M151A1 Jeep, dating to the AMC era, with horizontal grille bars. The jeep is somewhat rare as the military destroyed many of them at the end of their service due to the instability of the early four-wheel independent suspension they had.
This jeep is painted and equipped for Air Force Security, and assuming it actually saw this duty (many museum vehicles are unfortunately refurbished to appear to be of a different function than they actually saw), then it would have been used to provide security to the 18 silo sites in the vicinity of Davis-Monthan.
The windows and interior were dusty enough that getting interior pictures was impractical, but the plaque with M151A1 among other stats was barely visible, along with the plaque with the four-wheel-drive and gearshift controls. The interior was very spartan; the seats looked like bags suspended on visible metal frames, the steering wheel looked almost unchanged from the first World War II vehicles, and the floors and the interior were simply painted metal. Up close with an enclosed body it looked like the postal jeep's mean older brother.