Just looking at the S letter next to the Platinum - apparently the Japanese word for platinum is Shirogane so wondering if the S letter stands for that... It can't be S for silver and the only other possibility would be a maker's mark.
Testing Platinum by: Anonymous
Good Afternoon Richard
thank you for the crisp Platinum hallmark photo. Marks for Platinum include, plat, plat 10% iri, platinum & 950.
Although it was discovered in the 16th Century it was not used for jewelry until the 1800s and during WWII platinum was restricted for use as a catalyst in munitions plants.
The weight of platinum is a good indication of the metal, it's relatively heavy compared to the white metals, white gold, and silver. To confirm use a platinum solution and a touch stone. If the acid does not dissolve the mark on the touch stone the piece is Platinum.
