Two things ...
Bibelforscher is also what you will see next to the "purple triangle" symbol in Auschwitz and the like. It goes back to the pre-early 1930's Jehovah's Witness name declaration by Rutherford. "Bible Students" continued colloquially even among JW's rather longer in Europe than USA.
Also the date of the letter, 21 July 1944. It seems strange that a rather mundane letter like this is being written by Himmler to Kaltenbrunner the day after the unsuccessful bomb plot against Hitler by Stauffenberg and co.
These two men were at the forefront of rounding up the conspirators/backers of the plot and would have been right in the thick of the aftermath when this letter was purported to have been written.
I suppose it could have been dictated/penned a day or two before it was sent but I would love to see the original.