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

[Norme]

Hi Jonah, As you surely must know, the early 1920's were a time of great social and political turmoil in Germany. What ever marking regulations were in force at the time, "ordnung" frequently took second place to expediency. The fact that 1920 was both a date and a property mark only added to the confusion, and anomalies abound. Here's a P08 Navy (made in early 1914), that was marked 1921 by some local armorer. What was he thinking?
As to value, while a 1917/20 marked chamber is certainly rare, people who collect such oddities are rarer still, and I don't believe your gun will sell for more than any other unit marked 1917 Erfurt in similar condition. Since the gun is now listed on Gunbroker you'll know soon enough.
Regards, Norm