"Quick and dirty" means "there's a lot going on with many exceptions, but chances are, your problem will be solved with the following". Caveat Emptor, your mileage may vary, etc.
Greg's very familiar with Commando heads, so we can assume that he'd recognize one of those.
In this case (and most), the 5/16" vs 3/8" bolt holes is a reasonable "if/then" to identify a pre-Commando Norton head. Virtually noone is reducing the holes to 5/16", so that would have been (was) an easy one, no? Sometimes the flow chart will need to be longer than other times in the search for the telltale, but we'll get there.
With 50+ year old used motorcycle parts, anything could have happened, so possibilities of twin plugs, bathtub combustion chambers, 38mm inlets, or clamp-on exhaust spigots are as relevant and potentially confusing as any, if we're opening up the possibilities to any Norton head with modifications. Even in those instances, the 5/16" vs 3/8" if/then would still be an ok first step for narrowing a Pre-Commando head, particularly without a photo.