I cannot find if this platter is sterling or plate with the hallmark. It tested sterling, have not cut and acid tested though. It is only stamped with Silver.
My best guess would be Siam, modern-day Thailand, just based on the overall design and the "SILVER" hallmark is more reminiscent of the fonts I have seen on Siam silver, so I am leaning towards Siam as the country where this piece was manufactured, sometimes you will find Siam silver with just "SILVER" stamped on it although more often than not it will say "SIAM SILVER" :::