The key to the grade is on the reverse. I will give a second hint. MS. Eagle and arrows is where the grade is determined. and I love the MS-Stratosphere comment. I love this mini set of 1950s toned quarters that I'm putting together.
MS-66 folks are CORRECT! There are two bag marks on the eagle. This is a mint set toner... I am almost 50% through the 50s date by date and mint by mint for my little toner set and this one has really nice in hand eye appeal. I love those blue reverses!