In the '50's the Dutch Postal Services and the Dutch Railways placed a large order for "Plan C" rolling stock. Besides several series of carriages and streamlined multiple units, 10 coaches to transport mail were ordered.
https://nl.m.wikipedia.org/wiki/Plan_C_(spoorwegrijtuig)
These coaches ran as additional cars in passenger intercity trains. The staff of the Dutch Postal Services collected and delivered mail sorted during the ride. During halts the general public could post mail in the letterboxes fitted on both sides.
They were delivered in 1952. The last ones were withdrawn from service in 1979.
This is my interpretation in Gauge 1 (1:32 / 45 mm) of these beautiful coaches operated by the Nederlandse Posterijen and the Nederlandse Spoorwegen
03APR2023 - Uploaded revised set of 3D-files.
The author marked this model as their own original creation.