NJ Type design retains the rollers within the outer race, allowing easy access to the inner race for removal from the crank in order to shim the crank endfloat. If anything a far better choice of bearing than the original (an NF Type) we've all damaged brand new bearing cages trying to get that puller to work with the rollers in the way. If you have an NJ to hand, that's ideal, just the job performance wise and will be far easier to set up than the original stock design.
Lots about this bearing, its fitment and pitfalls together with facts and choice of other types available already on the Forum. 67 670 is the original BSA Part no. used on all shortstroke engines.... start to finish.
Swarfy.