Kernel Crittenden
US Veteran, Absent Comrade
Since private handgun ownership is forbidden in Japan, what you're likely seeing is ads for replica guns. Exact copies in every detail, they even "shoot" replica ammunition that looks just like .357 Mag cartridges. They also copied the s/n under the thumb piece, except every replica uses the exact same number..... if you google Smith and Wesson Japanese Police Revolver 360J some model ones that are for sale in Japan will pop up.....
The M360J's sold in the US today are clearly NOT overruns. They are purpose built. Everyone and his brother has one, they have been sold in large numbers every year for the last 9 years, are extremely common, easy to find, and very low priced.
If some sharp businessman found a way to import those funky Japanese grips to the US I think they would sell very well. They would fit the M360J and any similar J frame.