Sparky, I've always seen them listed in catalogs as "pony cleat" lampholders.
They're still manufactured in both plastic and porcelain. Even Home Depot still has them (the plastic version). There's a safer version also made in porcelain where a threaded collar screws around the screw-shell and covers the terminal screws.
I still daydream of finding an appliance or a lamp in a junk/antique store with THIS plug still attached to it!!
Second photo also from Kilokat's website