Durch Zufall bin ich heute auf die Literaturstelle gestossen, die alle gesucht haben:

Jones, Railway Parcel Post Cancellations ob Belgium, Belgian Study Circle, 2003

Es heisst auf Seite 58: The format was day/month/time/year, The month usually in words (...) Time gave the hour using the 24-hour clock. The year consisted of the last two digits.

Auf Seite 60 heisst es: One other development to be mentioned is the reversal of the positions of the time and the year, so that in most canceallations after the late 40's the time comes last. However, this had taken place much earlier in a few stations.

Der Übergang von Tag/Monat/Zeit/Jahr zu Tag/Monat /Jahr/Zeit war also fliessend.