Ludwig, Castello pipes are graded in batches, as opposed to a "set chart" of size designations or past historical records of grains.
When I visited the Castello workshop in 2019, I selected a group of pipes for the website. The smooth pipes were shown ungraded, which is the norm. After the dealer selects pipes Franco then grades the pipes as a group.
I asked Franco if I might offer my impressions on the grades and have him instruct me on my errors, which I expected the errors to be numerous. I did that and I was truly amazed at little aspects that Franco noted that I had not.
While the grading, to the untrained eye, appears to be random, to the eye of expertise, it does make sense.