This gasket is the large single piece oil resistent gasket that was fitted on the later OHV design from 1956-59 design. As you can see - the rocker mating faces had a quite different shape to the earlier OHV design, and this gasket incorporates the top of the pushrod area within its casting.
This rockerbox design covered all OHV variants from 1956-59 - so should fit Model 50 (350cc), ES2 (500cc and Model 19S (600cc) models.
We have included a diagram from the 1957 Spare Parts List to show its fitment (and will shortly be adding this 1957 Spare Parts Catalog in our Facsimiles library).
This oil resistent gasket is manufactured for us by a well established UK gasket manufacturer