Well, "Earthtones" got renamed into Tortoiseshell at IoWSG, where MH was just continuing with the things he'd been experimented with at Mdina, so there is actual physical evidence,

and it's in Mark's book about Ming.
We need to remember that studio glass was new, and basically "run by a bunch of disorganised hippies", nothing was standardised or catalogued.
Michael Harris just wanted to see what he could get glass to do. Once something was made, he lost interest and was on to the next thing. He didn't like signing stuff, he did like making glass.
They were short-staffed, they had trouble keeping up with making enough stock for the shop shelves, it sold so quickly.
The sort of finish on John's pot was used by the Ysarts in both Monart and Vasart with the strengthening extra blob, many, many years before Mdina.
