Answer

Manufacturing of clamshell Go stones dates back in Tokyo about 150 years ago and later in Osaka. Until then, other thin clamshell harvested in the neighboring area of Tokyo or Osaka was used to make Go stones. The thickness of the stone at that time was 4mm to 7.5mm.
About 110 years ago, Go stone manufacturers in Osaka happened to know that there were good quality thick clamshells in Hyuga (Miyazaki) area. First, the clamshell harvested in Hyuga was sent to them to be processed in Osaka. Later, some Go stone craftsmen moved from Hyuga to Osaka or some people in Hyuga went to Osaka to learn how to process Go stones using Hyuga clamshells. They came back to Hyuga several years later and opened their factory or workshops in Hyuga. This is the beginning of clamshell Go stone manufacturers in Hyuga. My grandfather worked as an apprentice for a famous Go stone craftsman in Osaka. I have heard that until the first 20 years from our firm’s foundation, my grandfather delivered all of his works to 1 Go dealer in Tokyo and 1 dealer in Osaka, total 2 dealers. So he made the shape of Go stones according to dealers’ preference or users’ preference. That shape was HoninBo-shape. At that time, the price of thick Hyuga clamshell stones was as expensive as one house. Of course, the price is a dealer's retail price, so I believe my grandfather, who was a hick, sold these stones to dealers at 1/4 of that retail price.

In 1960, my father went to Tokyo, being invited by a dealer in Tokyo, to check the high quality "Go stones made by my grandfather" owned by a rich man. Father said to me the owner of the stone lived in a huge house in Tokyo and he was a very rich man. There my father saw a great Hyuga Kaya board and beautiful Honinbo stones, probably Snow grade size 39.

Today, we have received old go stones from Go players all over Japan and have been asked for its repair. I perception is that Go stone shape have not changed a lot just like chessmen.