The breather tube was supported by a P clip on the back of the primary cases.
The very first time the primary cover is removed the P clip is either taken off, falls off or is not not bolted back up again.
The weight of the tube eventually causes it to foul on the chain when the chain will shorten it all by itself.
BSA followed the existing theory of the day to create a slight negative crankcase pressure when the engine was running.
And this theory dates from the time before oil seals when slingers were used to stop engine fumes and oil being pumped out from around every shaft.
The logic behind this was air would be forced into the cases through any holes.
It is only a real problem if the rise and fall of the pistons do not cancel themselves out ( single cylinders , 360 deg twins etc, etc, etc)
No one has really questioned this till receintly when Rex Bunn had a good look at things and has turned most "old theories" on their heads by finding that allowing atmospheric pressure to be drawn in on the up stroke & expelled on the down stroke actually improves the efficiency of the engine .
Through breathing is not new, it was used on a lot of old time racers but not on road models to any extent .
I am not trying to spruke his breathing kits but the theory behind them and some of the findings on the way through were quite interesting particularly on the way rings tend to work much better in one direction than the other, the resonant frequency of flapper type breathers ( as fitted to M & B series BSA's) , response times ( and lag times ) of commercially available one way valves ( PCV valves & Brake valves ) .
His work has caused both HD & Enfield to modify their engine breathing systems and some historic racers have claimed up to 0.5 increase in Hp, better fuel "economy" and less oil consumption.
Now back to reality.
The gasses in your crank are air, fuel & oil droplets ( caused by mechanical splashing inside the cases )
BSA attempted to remove the suspended oil droplets by creating a convoluted path for the exscaping air which would allow the oil droplets to condense and return to the crank. Good idea dosn't work at elevated revolutions.
Remember BSA published an oil consumption of 100 mls of oil per 200 miles for the A7 & A10 and this oil either goes out the pipes or out the breather or in reality out both.