Hi,
Uh-uh, the crankcase vents into the primary through the drive-side main bearing. All triples and '70-on twins, then the primary case works as a plenum chamber, the air flow slows, oil droplets vented from the crankcase should condense and fall to the bottom of the primary. Plus, as I say, the oil tank vents into the primary and, certainly on the OIF, under braking, oil could run from the frame down the vent hose into the chaincase.
Chain primary, the bottom run must be through some oil, to lubricate the chain and it splashes the oil around to lube the clutch rollers. However, you don't want so much oil in the primary it reaches the spinning alternator rotor causing power-sapping drag. The three holes are to allow excess primary oil to drain into the crankcase before the level reaches the rotor,
not to vent the crankcase into the primary.
Ime (small), the three small holes are in position for a
belt primary? Then you don't want its bottom run through any oil, holes that low allow
any oil that enters the primary to drain straight out again into the crankcase.
That would've been the original timed crankcase vent (as on your other engine?); the crankcase vented through the hollow inlet cam, which drove a vent that was open when the pistons were descending and shut when they were ascending.
Proverbial 'solution in search of a problem'? As I posted earlier, primary simply venting into a hose connected to the filler hole, hose routed to the back of the bike, the crankcase will simply blow and suck mostly the same air (in the primary and the hose), just the pressure pulses will go to atmosphere.
In the 1960's, before Meriden changed the twins' crankcase venting, leaving out the drive-side main-bearing seal and drilling the holes between crankcase and primary was such a popular 'tuning' mod. in the US, at least one of importers marketed an accessory screw-in filter to replace the standard primary filler plug.
Educated guess says:-
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View attachment 800029
feeds the engine from the frame;
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View attachment 800030
, one is the return from the engine to the frame, the other is the frame vent?
Mmmm ... as above, long before Meriden did it officially from late '69, crankcase venting into primary rather than through the timed breather was a mod. by racers, that spread to wannabes; GB had similar, with (before
Easy Rider) owners aping road racers, bolting 'clip-on' handlebars half-way down the fork legs, gearshift and rear brake levers by the pillion footrests ...
Hth.
Regards,