Mbinsewi got it right, immediately.
Worey's book Iron Horses of the Santa Fe Trail (SW RR Hist. Soc., 1965) says these were all 1500 h.p. GP7's built in April, 1953. None of them was a slug. They all had prime movers, but the ones with "A" suffixes were cabless boosters. They all had dynamic brakes. They were delivered in Santa Fe's classic black and silver scheme.
They were part of Santa Fe class 2650, which ranged from 2650 to 2893, and 2788A to 2792A (244 cab units and 5 cabless boosters).
The cab units weighed 249,000 pounds and had a tractive effort of 62,250 pounds. The boosters weighed 247,150 pounds and had a tractive effort of 61,790 pounds.
With the delivery of this massive order during 1950-1953, Santa Fe took a giant step towards total dieselization, and the last steam runs were made in New Mexico in August, 1957.