Notice the difference in the head angles

VS second series
The Snap on Angle Head Open End

The openings are angled at 30 and 60 degrees for limited access areas. This allows for 4 angled to grip and turn. Both sides of the wrench have the same size. This 30 – 60 combination, with a flip gives the user 4 angles to work with. It is designed to turn nuts in a narrow swing area where other wrenches can not gain enough of a turn to work. They have rounded edges for fit in the hand.  

There are two series of these. The first series has recessed handles and existed from 1952 – 1970. How much before or after I am not sure, but they did not exist in 1946 and after 1971 they second style existed with a polished handle and no recessed area. 

The first picture is the 1952-1970 style, the second is the 1971 – 1999 style