Nice project Andy!
Theses "curious containers" above are called "pa" (porteur aménagé) or Grossbehälter/Mittelcontainer. Invented by the Netherlands in the 1940-Years, that system was based on common standards. To transport it, it was needed to modify flatcars with rails and holding spindles.
The exactly same container in the SEK-pictures above was also used in Switzerland and Netherland. In 1:87, tese are available by Artitec: http://www.artitecshop.com/de/satz-3-geschlossene-containers-ns.html
Lather, the greek-built ones are different in shape. I have explored some of it with BarpaP in Rentis. Last year, i spent a day of research in the SBB-Archives. From theses "NL"-Type, there is unfortunatly no information. So i have begun with designing the next generation of pa-flatcars from 1962, similar to DB BTmms 58 or the OSE-Flatcar.
