Not a U6B, but close - Vietnam got U8Bs presumably bought at least with assistance from the US Government.

Built in three batches between 1963 and 1965, s/n 34827-49, 35370-9, 35408-22 for Vietnam Railway #BB907-BB954 (48 units) Caterpillar D398 V-12 engine 900/840 hp renumbered to D9E-207 to D9E-254 in 1975. GE records show these as 57-58 tons which is about the basic weight, though many customers chose to have units ballasted to various heavier weights. Recent information suggests that many of these are still in service in Vietnam, including a few that have been upgraded with new Caterpillar 3512 engines in recent years these being reclassified as D10E but keeping the same suffix unit number, presumably around 1000 hp.

The 1956 U4B/U6B was advertised by GE with a drawing that is very close to the USG 54-tons except using export style handrails. The 1959 built U6B resembled the USG units only in some details like the close clearance cab, but reverted back to a vertical radiator exhausting through the roof like the 50/52/70/95 ton designs with a slightly different hood. The truck design went to a heavy single drop equalizer with a single row of heavy springs. The successor U5B/U8B models reverted back to the USG type radiator (low intakes, horizontal fan driven through right angle gearbox with overhead radiators). Many U5B/U8B got the odd rubber swing bolster truck, though the Vietnam units were among the first to get the cast floating bolster truck. The back of these cabs were flat and as far as I know most if not all were long hood forward (except for 52 tons in Costa Rica).

On the successor U6B/U10B the cab end became the front of the locomotive and the end became angled instead of flat. There were some minor body variations, and the cast floating bolster truck became standard except the single drop equalizer truck was still offered as a lower cost option for locos restricted to switching or low speed service. In many cases the changes between models are very subtle and in some cases there are no visible differences between the V-8 and V-12 versions, so it is not always possible to definitely identify models by sight.

Being moved upriver by Navy ship is an interesting story.