Anyone seen this stone before???

Hey guys,

I received a stone from my (rather bearded) father, who received this stone from my grandfather. In his early days as a cop, he (my grandfather) got into straight shaving through an old Japanese lady barber. She ordered him this stone straight from Japan and (according to my fathers story) this is the only stone he used to hone and shave with.

I tried to research it, but came up empty handed (mind, this was back in the 60's, 70's era). I have no other stones to compare it to for grit (besides barber hones and the coticule/combo strop). Anyone have an idea on Takumi stones?