And last time I checked, open cleat lampholders were hard to get and somewhat pricy.
They're available at any Lowes or Home Depot for only a couple of bucks. Ditto local hardware stores, you can easily find them there. I think I have two in plastic and two in porcelain.
I wonder what they'd be used for now? Guess the exposed terminals come handy when you're using it as part of some home-made test equipment.