I don’t have specific information. However....I tried a Google search for Sekishu pottery. If you haven’t already.....You might try that also.....there’s some reading on Sekishu pottery....I don’t know if it will be helpful or not...just a starting point. Content from one point...
https://www.select-japan.net/ex/shimane-sekishu-kameyamayaki/
About Sekishu Kameyama-yaki
Sekishu Kameyama-yaki is a form of pottery that – rare for pottery nowadays – is produced entirely in-house; from the collection of clay on our home mountain, to the shaping, to the firing. It has also been designated a Hometown Traditional Craft of Shimane Prefecture. Day in and day out, we devote ourselves to our craft to make sure every single piece we produce embodies our philosophy of “Relief through sight, touch, and use.”
Since around the turn of the millennium, we have created many carbonized yakijime vases and beer cups. Carbonized yakijime is a process in which pieces are fired over an extended period of time and discolored during the process, resulting in unique works of art.
We make pieces for wedding receptions, private family parties, senior birthdays, Mother’s Day and Father’s Day celebrations, baby showers, and company founding anniversaries.
I have no idea what you have....this was just one link.