Bat grading system is a scam and buyers are gullible enough to pay for it. All bats should be graded on a number of measurable criteria (willow density, dead weight, swing weight, location of node of percussion, swell position, balance point, handle stiffness, handle thickness). We also need to measure the rebound generated off the surface of the ball. From my discussions, I suspect James Laver has some "secret" method of determining quality/ping of a timber before he starts shaping a bat. Whatever that is, we need a way to measure it and standardize. Look at tennis racquets, golf clubs. It is practically a science in those sports but cricket is a stone-age sport with bright stickers.
Bat sizes as we current know them are wrong too - many decades (or centuries) ago, someone arbitrarily drew progressively larger bat sizes on a piece of paper and THAT became the standard. Na-uh uh! Through trial-and-error, we know that a standard SH length with a shorter (than SH) blade generates better bat speed. In general, people use bats that are too long for them. Size SH is too long for many players. Size 6 or Harrow should be the standard bat size.