3 1/8" X 3 1/4" (78mm X 82mm)

(skirt ink stamp)

{NGK}

Picture courtesy Bob Berry

3 1/4" X 3 1/4" (80mm X 80mm)

(top incuse)

{maker unknown} 

Picture courtesy Craig Polkinghorne

3 1/8" X 3 1/8" (77mm X 77mm)

(ink stamp on top) MADE IN OCCUPIED JAPAN 

                         around / 48

{maker unknown} 

Picture courtesy Bob Scafe