Hi, your description is not too clear, its hard to identify exactly what you mean.By "right hand spindle" I would take that as the main, long spindle 42-6320, as right and left components on a motorcycle are generally referred to from the point of view of a seated rider.
By right hand spindle, I assume you mean the LEFT hand, short, spindle 42-6325? If this is the case, I think I can help. I do not think you have anything missing, and I agree the parts book is a little confusing, but bear with me, bit long winded.
Firstly,the "cap" 42-6328 is nothing to do with the short spindle or its bearing, it is simply drawn near to them in the parts book, but it is a knock in tin ring/cup that fits inside the main hub casting as a grease retainer and centraliser for the main, long spacer 42-6323-you can see it if you look inside the main hub casting from the sprocket end,between the four studs.
Thats simple enough, the next bit is harder to grasp, and I feel you are puzzled, just as I was when i first looked at this assembly! The BSA rear hub is not entirely logical in its design-I think its a bit of a dogs dinner at the sprocket bearing end. There is meant to be a gap between the inner race of the sprocket bearing (89-3022)and the shoulder on the innermost end of the short spindle 42-6325. This bearing is a press fit into the sprocket, and is pressed in right up to the shoulder in the sprocket bore. Once in place, the rather flimsy tin cup"retainer" 42-6105 is knocked into the sprocket bore so the bearing is retained both sides. 42-6105 should be a tight knock fit into the sprocket, if loose-new one needed, it is retained simply by its tight fit (see-I said dogs dinner!).- easy so far.
The short spindle 42-6325 is then tapped (may slide in loosely anyway due to wear)into the bearing through the middle of 6105, but there is no need to tap it in all the way to the shoulder on 6325, you could do, but its pointless. Now,we fix the sprocket assembly to the swingarm via spacer 42-6324, and washer/nut 6078/6077. Spindle 42-6325 is now rigidly held to swingarm .When the wheel is then fitted to the motorcycle, and the main spindle 42-6320 tightened up, the final job is to replace the four sprocket nuts and tighten the sprocket onto the main hub casting.When you tighten these four nuts,the sprocket bearing inner race will be forced to move along the short spindle 6325 slightly, to find its own natural place, according to manufacturing tolerances in the fit between the sprocket and hub faces. If there was a spacer or circlip on 6325(the one you think is "missing")it would stop the bearing inner from sliding to this natural place and strain the bearing as the four sprocket nuts were tightened up. In short, this arrangement (with your 1/8 "gap")allows the sprocket bearing to "float" a little along 6325, perhaps to allow for to expansion etc if the brake drum gets hot (ha ha!)
Hope this explains it- `least that`s how i understand things! Best Regards, Bob C