My daughter's mother-in-law descends from Jown's brother Frank. We've found out a few interesting tidbits. One of them is that John and Victoria had a son named Franz, who would have been 23 in 1920. This could be the Franz from the letters.
Also, Franz and John had three sisters. The youngest was Mathilda, born in 1879.
Have you learned anything more about Franz the elder and Anna Muller? We're hitting a wall with them. We may have found a reference to Anna's grave in Vienna, but the dates don't seem to add up.