These copies I remember from the 1960's.  They are great reads, even for adults.  They are in great condition.  The 99 cent thrift store price is inside cover, as seen in last photo.  Titles are:

The Great airport Mystery [1965] #9

The Twisted Claw [1939] #18

The Missing Chums [1962] #4

The Mystery of the Flying Express [1941] #20

Four vintage copies of the Hardy Boys mystery thriller series by Franklin W. Dixon, published in the 1960s by Grosset & Dunlap. The books are hardcover and feature the popular children's book characters.