WIA GC 1930/3582
Bing, Gertrud to Warburg, Felix M. 15/07/1930
WIA GC 1930/3583
Bing, Gertrud to Warburg, Felix M. 19/09/1930
WIA GC 1930/3585
Warburg, Felix M. to Bing, Gertrud 16/10/1930
WIA GC 1931/3302
KBW to Warburg, Felix M. 13/01/1931
WIA GC 1931/3303
Warburg, Felix M. to KBW 27/01/1931