This is a good quality aluminium type head gasket - of the type that was fitted to Sidevalve Norton engines (i.e. 16H and Model 1 types) on the 1948 - 55 type engine.
This later type side valve engine was based on the crankcase re-design that took place in 1948, and was identifiable from earlier versions, in that it had a seperate alloy casting for the tappet adjustment area.
I do not have an engine of this type in my OHV/SV parts collection any more (although one of this model was the first motorcycle I ever built and owned!) - but have included a picture from the 1948 Spare Parts catalog, to identify the head gasket from the earlier copper side valve head gasket fitted to the pre-48 models, that we also stock (item 0925).
This gasket is manufactured for us by a specialist and well established gasket manufacturer in the UK.