Vintage American outlet

[bgadow]

I posted this on ARF a couple years ago-never did get a good answer on why it was done this way: the old part of our shop was built in 1950, maybe earlier. All of the outlets looked like this:

All were 120vac. I eventually switched them over to modern outlets. My Dad had coped with these by simply twisting one prong on every plug in the shop. When he added on he bought a bunch of ground adapters to make it easy to switch back and forth. Any ideas why they would use these?