Correct (and as per original) 1/4" BSP 'One Way Valve' - as fitted to all Norton singles, including all SOHC, OHV and SV models - this item screwing into the recess just above the drive side main bearings, allowing the engine to breathe correctly
This special 'one way' valve' looks almost identical to the 1/4" BSP 'straight through' breather that was also fitted to various Norton single cylinder engines, but serves a very different purpose - it being fitted just above the driveside main bearings, and allowing the engine to 'breathe out' but not ''breathe in', which is achieved by the use of small bearing fitted inside the valve.
This valve was fitted to all SOHC (and DOHC) crankcases (as well as all OHV and Side Valve models) behind the drive side mainshaft casting. On original engines this special valve is almost always badly chewed engines and is now almost impossible to find. They were also fitted originally in bare brass, as shown. This version, like most items I am reproducing, has been manufactured to be indistinguishable to the original, and as normal in this case an original was used as the pattern.
The outer breather union is machined to the same Whitworth spanner hex as the original, is plain brass and has the same thru-hole size as original. It is threaded internally to take a small screw in insert, which retains a small ball bearing (in stainless, to prevent corrosion), which provides the one-way breather mechanism.
These should be fitted with the inner screw in section showing uppermost (just in case you are not sure), to ensure the breather allows pressure to escape out - not the other way round. I get asked by lots of single cylinder Norton customers why their engine may be leaking more oil than normal or not oiling properly - I always direct them first to check if this one way valve has been replaced by a 'straight thru' breather by mistake, or even worse - it is fitted, but is screwed in the wrong way round!