Vintage style RCA AD-1 microphone stand adapter 5/8 X 27 to RCA thread 77DX # 4
As seen in pics # 6, 7 & 8 this adapter fits on any desk stand and then allows the BK5A model, 44BX and 77DX model mic to be mounted. You are looking at an unused AD-1 microphone connector / adapters used primarily on RCA & Electro Voice model microphones but also a few Japanese ribbon mics.