Appearance seems utterly random, with the exception that I've never seen grooved blond wood. I've had a '43 with the grooves, a '42 without, & a blond '55 without.
I vaguely recall a thread at Gunboards about the grooved vs non grooved. No one could post a pic of grooved Beech wood guards. One or two claimed to have seen them at one time, I never have. I don't recall ever seeing a MkII with grooves (original).