any information about this 1917/20 unit marked erfurt would be appreciated

[Ron Wood]

Well Jonah, this is your lucky day. It would seem that Klaus feels that in his experience the 1910 directive probably did carry over into the Weimar era, therefore the "1917/20" would indicate completion of a 1917 piece was delayed until 1920. Now as to why that happened I have no idea (but then again I evidently had no idea that it wasn't a property mark, so I am 2 for 2 ).