Hi Dianne,
I remember seeing a Japanese egg cup with Y.S. mark in blue. It was just like all the other mid 20th century Japanese willow with various marks from different makers. There would be no difference in value to pieces by any of the makers or even pieces with just "Japan" or "Made in Japan". Moriyama is a higher priced Japanese willow maker mostly because of the unusual pieces made such as covered boxes. Also a 20th century maker.