Remember it was the high time of the cold war. Some strange things happened at that time, even connected to trains. E.g. the Deutsche Reichsbahn of the GDR refused to accept waggons which were renumbered to the new Bundesbahn numbering system without Direktionsnamen in the early 1950s. That's why many freight cars still got Direktionsnamen until the mids of the 1950s. Similar the indicator of Brit.-US Zone. This was something like a workaround to the problem above. New numbering, but Brit.-US Zone which indicates that the Bundesbahn and the Bundesrepublik Deutschland are not independent (whether this was really true or not would be a nice thing to discuss while having a nice glass of beer or coke on a Saturday afternoon :-) ).
But having such indicator in 1957 would really be something extraordinary, but not impossible.

A little bit off site:
The DR of the GDR renamed all freight cars with Direktionsnamen of then Polish cities, like Oppeln to Oschersleben (?), Bromberg to (?) or similar. Think of a train with a DB Oppeln next to a DR Oschersleben. What might the train worker have thought about this? Especially when he was expelled of former German now Polish parts of Europe.

Martin