From whatbi can rwmwmber Iwasaky Kokusen did reaserch on the asano naguras and some well renowned mines like nakayama. Kokusen had great ties with the owner of the Asano shiro nagura mine and helped implement grading of the stones.
He also had ties with the former and curent nakayama mine owners.
Shigeioshi is afected by senile dementia or Alzheimer disease and is sometimes hard to talk to his memory is not what it used to be.
I remember that when Kokusen started making tamahagane razors some barbers had trouble honing them and blamed it on the razor...Kokusen had to research the way barbers honed and the tipe of steel and temper...geometry they prefered...he soon found out that many of the barbers did not know how to hone properly...using far to much pressure or not using the stones corectly.
Soon after he wrote his booklet ''Honing razors and nihon kamisoris''
It was after this that these stamped marukas made it to the market.
There are also large bench size stones stamped iwasaki and maruka...not only naguras.
They sell for a small fortune in Japan.
Word has it that Iwasaki Kokusen hand picked the stones at the mine and Kato stamped and boxed them.
I dont think they were made for marketing but rather to provide a sample of quality of what stones should be used for his razors.
That's what I know from reading here and there over the years...I dont know if it is all true but I belive it is.