CJ you are correct. The 5511 that pulled #68 out of Alamosa on 11/4/56 was on FT A unit. The FTs, F3s, F7s were purchased as 4 unit sets, 2 A units and 2 B units. The sets were numbered, for example, 5511 5512 5513 and the 5514. the two A units were numbered 5511 and 5514, the two units in the middle, the B units, were the 5512 and 5513. All RG A units end in 1 or 4 and the B units all end in 2 or 3. Similarly, GP 9s were numbered as 4 unit sets ie 590-1 which looks like 5901 and 5902 5903 ETC The next set in the sequence would be 5911 5912 5913 5914 and so on.
I know this is a narrow gauge/steam list but this stuff connected with the narrow gauge at Alamosa, Salida and Montrose. All this equipment, SG and NG, was in the same yard at the same time. Alamosa was a very interesting place in terms of railroad for several decades.
Woody