I had no idea helmet specs weren't universally accepted across the board. Beyond Snell, yrs truly has heard of the Jim Clark Foundation specs , but obviously there are others that may or may not conform to one another.
My suggestions would be buy the latest updated Snell from Jegs, Speedway, Summitt, etc. or Chapparral, for instance, if you're a bike guy, and hold the salesperson's "FEET TO THE FIRE" (Quoting Jack D. - 2004) if they send you old stock.
In other words conform to the race group's specs in which you compete.