Well this is only a guess but- First the cups are easy to tell which side they go on. The LH threaded cup goes into the drive side (as the BB is for a BSC shell). Then the axle/bearing unit is installed. Usually if there's a difference in axle length, extending past the bearing unit, the longer side goes into the drive side cup. then the second, RH threaded, cup goes into the non drive side of the shell.

It's easy to assume that the grooved end of the unit matches up with the grooved cup. But a measurement should confirm if this is not wrong. Ultimately there's no wrong way as long as the crank arms/rings are located the proper distance from the frame's centerline and chainstays. Dry fit first then while leaving one cup in place remove the other, lube/anti seize/lock tite (your preference) the removed cup back in place and repeate with the second cup. This way the axle/bearing unit it kept in the same location while the final cup installing is done. Andy.