The Novoflex LEIMIK CRMS Microscope Objective Lens to Lecia Screw Mount Adapter is an adapter which allows you to use any microscope objective lens adhering to the Royal Microscopical Society standard upon the Novoflex BALPRO1 or BALPRO T/S bellows systems. This is a 0.8" diameter, 36 threads per inch size. Modern microscope objectives from Nikon, Olympus, Zeiss, Leitz, etc. all adhere to this common standard.
The BALPRO bellows are larger bellows which also accept medium-format cameras and lenses in addition to 35mm system cameras and lenses, or in this case a tiny microscope lens. As the hole on the BALPRO is large enough to accept a medium format lens, when you're using it with a 35mm system lens (or a microscope objective) you must first reduce the front of the device to the Leica screw mount standard by installing the Novoflex PROLEI adapter. From that PROLEI, with its incumbent female Leica screw mount threading, Novoflex makes a host of adapters to use other lenses-such as this LEIMIK adapter, which allows you to install the microscope objective lens.
The fact that you're using a microscope lens has little to do with the rear of the bellows; you could use any 35mm system you liked, or even perhaps a medium format system if the bellows was extended far enough so that the objective's field of view began to cover the image capture are of the medium format camera (or medium format digital back).
