this is a lid for the tupperware lettuce keeper called the crisp it.
it is for the smaller of the two crisp it bowls that Tupperware makes.
they also make a super crisp it.
this is the smaller one, and it is 7 1/4 inches across and has a bump out in the center, to fit the shape of the lettuce head.
lid is used, has discolored over time, not as bright as when it was new, but still seals air tight.