You are correct Pierre. The Plan D and Plan K carriages are build from the same design. Plan D was ordered by the Dutch RR just after the war and Plan K was ordered and build in 1957/1958.
The main differences between the two can be found in the interior of the carriages, such as the existence of 3rd class compartments in the Plan D carriages.
The site by Martijnhaman (
www.martijnhaman.nl ) is an excellent reference for this kind of information. I can however understand its a bit difficult to read for non-Dutch people. If you need any info translated from the site just send me a PM and I will gladly translate it.