Ok so, I took the FatBoy apart, and inside is a lot of brown powder.. First I thought it is rust, but then I realised it is paint. Researching, I found out that the microphone I have is one of the first Fat Boys released. They had the interior mesh (made of silk) painted in red, (which apparently turned brown after all these years..). The later models had a switch at the back (to select frequency empedence) and later again they released a much smaller FatBoy microphone which then became one of the most widely used microphones in radio broadcast etc. So I can safely date this mike pre 1946, which is when they fitted the switch.