I’ve had two RSM setups using the Black Tiger motors, the .35 size and the .40 size. That extra bearing never got used on either one. I still have them in my box of bits and pieces.
The .35 size motor is on a Magician, rear mounted, that I flew for a while, then sold. The new owner has put a bunch of flights on it without any problems, it’s still running strong. As I recall, with the rear mount, there was some shaft that protruded to the rear through the mount. In that instance I cut the protruding shaft off with a Dremel wheel.
There are people on here, Eric Rule being one, that say using that extra bearing is necessary, and not using it will result in motor failure. There may be some truth to that. I had the .40 size on a Primary Force. There was space for that shaft to reside in and it spun happily in place. That motor was always noisy, and after many flights I ordered bearings. When I replaced them I found one of the shields boogered up. The motor was only slightly quieter with the new bearings, and I decided to try a rear mounted Cobra motor. It was much quieter and it remains on the PF nose.
Cutting the shaft off is not too scary. If you decide later to use that motor in a front mount configuration you can buy a new shaft from several sources.