The "twins" are NOT the U4B model. At some point in the past, someone found out that GE first advertised a U4B and U6B model in early 1956, and based on the 390 hp rating incorrectly assumed that the USG units were the U4B model. The U4B specification listed by the DieselShop is horribly incorrect, and derives primarily from the Cooper-Bessemer powered 70 ton (which did not evolve into the Universal series), not the Caterpillar powered U4B/U6B.
If you look at their roster, notice they don't even include the USG twins.
http://dieselshop.us/GE_U4B.HTML I'm not sure what the 62 tons for Tela RR were, but the number series isn't right as they should be in the 600s. I've seen photos of the 722 and it is a center-cab. These were the common 65 ton design modified with Cat D336 engines (instead of Cummins) and for narrow gauge. ANd look at their builder's date - the U4B was offere in 1956 but soon dropped from the catalog due to lack of interest.
By all rights, USG should have gotten a pair of 50/52 ton units, but the GE747 traction motor design did not have the capability USG needed. The Porter had issues with their Westinghouse traction motors, turbochargers, engine cooling and its Cooper-Bessemer engine. (GE was still building the C+C 70 ton for narrow gauge like the US Potash units, but USG would not have been interested even though they had a GE 70 ton in Nevada). What little information does exist suggests that the USG twins were a custom design for USG using the 50/52 ton as a basis, adding GE764 traction motors, a close clearance cab, and a modified cooling system. Whether the USG units are a modified U6B or adopted by GE as something new for their catalog is up for debate, but there is no doubt they are at least a pre-production or prototype Universal type.
Neither the advertised U4B and U6B of 1956 was built. The USG units did not use the same model of Cat engine or main generator of either design. There was an intermediate U6B specification, but the first weren't built until 1959 (Santa Maria Valley #60 being one). By 1961, the U6B was replaced by the U5B and U8B as a result of Caterpillar changing their engine designs, and the model being upgraded by GE. 1965 to 1992 saw the U5B being upgraded to the U6B, the last being built with a newer Caterpillar engine design.
