You have mentioned the best way already, a copper gasked around the ridge is the savest, becouse if it's on the ridge there is a chance you will crack the head. This is wy 50% of the 4 valve heads are cracked.
The hunter had the quick turning throtle lever in 1936, the low barrel is 250 or 350 but the front mudguard seems hunter. On this photo it would be 5 years old, that would fit the shiny clutchdek. You can see it better on; nederlandsearielclub.nl - forum - algemeen. The number could be DDE22 or DOE22.