When I worked in Sioux City Iowa, there were plenty of old Sioux air tools around.  Several times I ran across an impact wrench in 5/8in.  The story I got was, Sioux built all their first impacts in 5/8 so guys wouldn't be tempted to use them on their chrome sockets and shatter them. Am seeing reference to SO 5/8 drive now, same story? 

Another story I've never been able to confirm; During the Second World War the government bought huge quantities of impact wrenches for both defense contractors and the military.  To prevent sockets and impact tools from leaving with workers they were built 5/8" drive.

I've run onto 1 SnapOn 5/8 impact with sockets still in the original cosmoline and government metal box, and a surplus dealers have a good number of them from time to time.  I've never been able to confirm anything beyond that.

The flaw in both theories is SnapOn was making plain 5/8 drive sockets in the 20's , so there were sockets available if someone wanted to mate them with an impact gun and demonstrate the socket-schrapnel-effect....

Souix was a contractor to the gov, so it's possible the 5/8 drive impact sockets were just to fit those 5/8 guns, only speculation tho....

I'm not sure the gov was as worried about field personnel stealing tools as the tool companies were worried about getting gov spec'd tools turned in for warranty service (stealing uncle sam's tools gets you an all expenses paid holiday in leavenworth).
Then again,  the black market was a booming business, so who knows...