Simplify the following text for a Grade 8 native English-speaking person.
Original: A plate block , or plate-number block is a block of postage stamps that up until the late 1960 's included two rows of stamps attached to one another in a block of four or more , with printing information , including the printing plate number , on attached margin paper .
Simplified: A plate block is a block of postage stamps that includes the part of the sheet margin with the serial number of the printing plate ( s ) used to print the stamps .