Interesting. The "Diesel Spotter's Guide" isn't, I think, entirely error-free, but -- in the absence of specific evidence to the contrary…
The original DSG was out of print by the time I … got interested enough in railroads to start buying books … so all I have is the "Second Diesel Spotter's Guide" of 1973. It has somewhat different wording: that after being built as export demonstrators, the first two were sold to N de M. (This revised wording has been preserved in Kalmbach's later "Diesel Locomotives: the first fifty years" (1995).)
For curiosity, what does the original DSG say about the U28B/U28C? I had gotten the impression that the change to the new carbody design and the change from straight DC to AC/DC transmission were simultaneous, but when I went back and checked I saw that the "Second Diesel Spotter's Guide" doesn't actually say that, but leaves open the possibility that (as is in fact the case) many units with the new carbodies had the old transmission.