According to my knowledge Maerklin wanted to start a 1/200 line in 1938 (similar to Dinky), but war was faster... After some issued DO-18, the toy production was ceased and the line dropped. Some prototypes of other models are still existing, see link below.
Even though the plane looks similar to Dinky, I assume, it was made in Goeppingen. During that time Maerklin was going full throttle ahead with designing zinc alloy casted HO trains to replace the tin based 0 gauge trains. So they had the capacity to mould die cast toys, as they did already for their car and military models.
Another Maerklin DO-18 was sold I guess a year ago on German ebay. It had the ink writing on the below rear flaps applied and went for around 400,- Euro (I gave up at 250,-).
Here ist the link for those, who are interested in Maerklin planes (tin and die cast). It's Swiss origin and German language, sorry...
http://www.tinplatefan.ch/WWW/LIGONET/k ... enDocumentPS: Take care! The Swiss author did the same mistake as S. Richardson in her book and named the DO-18 also "Wal"! This may cause confusion!