Suggestions for making the primary case oil tight

[Greybeard]

...an annoying oil leak from the primary case...

It's likely that if Allen screws have been used they will have been overtightened which is likely to distort the case and pull up the land around the threaded holes. BSA designed these cases to be sealed by no more force than can be applied by a screwdriver. There is really very little pressure in the cases; it should be easy enough to get a seal. As already described run a fine file across opposite sides of the case mating surfaces at the same time, (to ensure accuracy). On my Plunger model after thorough cleaning I use Blue Hylomar on one side of the gasket only and do not have leaks. I'm using ATF fluid in the primary case.

[BritTwit]

One tip I have not seen mentioned, is from an old issue of "Twitter" (GSOC news letter) from the '70s.  I personally tried this on my RGS primary which was warped, producing a 1/8" gap.
First remove any high spots and clean the threads.  Next, bolt the inner and outer cover together at approx. 25 ft. lbs.  Place in an oven preheated to 400 degrees.  Heat for about an hour and let cool.  The gap disappeared. 

[duTch]

 

  Hey Richard (Orabanda), is that like this,
http://www.ebay.com/itm/YAMAHA-Genuine-New-Motorcycle-Parts-Jog-Crankcase-Breather-583-15371-00-/282437249232?hash=item41c29268d0:g:1ZQAAOSw2gxYyjxw&vxp=mtr

  Called up the local Yaya dealer, and ordered the microscopic part, but it has a superseded number #3YJ-15371-00, instead of '#583-15371-00'  (just for information); au$4 buck smackerooos- can't argue with that but should've ordered one for the box too...

I drill into the back of the inner cover, just above the nut which secures the kick starter stop

 Good spot- never thought of that (obviously)