This envelope was postmarked on October 31, 1944.
Notice the vintage RKO Radio Pictures Inc. stamp in the upper left of the envelope. Boris and Bela were just beginning the filming of Val Lewton's THE BODY SNATCHER at RKO and would continue doing so into the early part of 1945.
I can't say that I have ever seen another item signed jointly by both of these horror legends. An 80 year old rarity!