There were two significant breakthroughs in the last 24 hours. It may take me a while to write them up fully, so I will just give the summaries for now. 

The collection of West Prussian address books linked in the last post includes a substantial number of directories from the city of Elbing, 39 years of them between 1820 and 1942. We know that our Wohlgemuth family lived there in the late 1890s. Isaac was listed as a mill owner living in Elbing on his 1898 marriage record. Our grandmother, Elly Wohlgemuth Ringel, was born in Elbing in July 1900.

Rodney Down Under came through again. After I showed him the marriage record for Rosalie Paechter and Jacob Kleemann, showing her father as Julius, Rodney took a look himself in one of his go-to resources, a collection of address books for Danzig and selected other towns in West Prussia. Listings of Tiegenhof are available for the year of 1858, and only that year. On the second page of Tiegenhof addresses was the listing you see above for Julius Paechter. 

I had expected to find the Wohlgemuth's mill building near to the Elbing River that flows north-sourth on the west side of the city. Instead the address books placed them inland to the east of the center city. 

I looked for a period street map that might let me identify the location of Mühlendamm 8/9. The image above is from 1911 and gave just enough clues to point to the answer. 

I discovered the vivid article about the arson at the Paechter store on the Geni web site attached to an elaborate family tree. According to the Geni records, Isaak Paechter, the storeowner in the article, was married to a Friederike Paechter, of all names. Their two children were Rosa and Kurt Julius, and the family later relocated to Berlin. 

Several posts back, I mentioned that Rodney wrote to introduce me to an Australian contact. Now I can tell you that he is Peter Nash, the author of Escape From Berlin, which describes the ordeal his family endured in their flight from Nazi persecution.