This is designed to hold three Transsound TSB-165A microphone capsules, and is meant to be used in conjunction with Jules Ryckebusch's Instructable: Build the MS Alice Stereo Microphone
In the build one capsule faces forward (up in the model), and the other two capsules face sideways - one facing left, the other facing right. The two side-facing capsules are wired in series with opposite polarity, and combine to create a figure-eight microphone. When used in conjunction with the forward-facing capsule, the three comprise a self-contained mid-side stereo microphone.
The hole pattern at the base of the saddle matches that in the Neewer NW-800 microphone body. It will likely match the hole pattern in other BM-800 microphones as well, but I haven't had a chance to confirm this. If you order one of these and install it in another BM-800 microphone, please let me know so I can update the description.
The capsules should be attached using an appropriate adhesive. I've used E-6000 silicone adhesive with good results, but hot glue, other silicones, or polyurethanes should work as well.