Yes it looks like a one way valve probably used a pollution control valve if it was piped into the engine breather to pull fumes into the engine to be burned. If the valve is just open to atmosphere then all it would do is draw air in and cause a weak mixture.
Just a quick note , sorry my deflector plate is steel , maybe stainless ? AND after cleaning it up and seeing ''FRONT'' inscribed , it was fitted the wrong way round Ken
The one way valve goes into the tin box on top of the carb , I can just about blow through it from the carb end but not the other way .
I have the 'spare manifold' apart and has good deflector plate if anyone needs it ? I may see how well the inlet manifold fits my exhaust and use it , if OK , as it only has the small flange hole which I can block .
Well done getting the manifolds apart. I managed to get some good quality manifold/head gaskets from F W Thornton of Telford. They look to have a metal type facing to both surfaces so am thinking that these are better than the cardboard looking ones. Any thoughts on that Panky. If you are looking for new brass nuts for the exhaust connection the only place I could find with the inch long ones that came off is www.scparts.co.uk., again a silly price but there we go. Spalding fasteners and Wrights Auto supplies list them but are out of stock.
Hope this helps, keep up the good work.