This appears to be an early Blacklight bulb. It is made of extremely dark glass (can't see through it), and it gets EXTREMELY hot while in use. It might've been meant for intermittent use. The electrode inside appears to have a "C" shape. The bulb might be filled with Argon gas. It has a near-identical appearance to a #22R infra-red flashbulb, although this bulb is for a completely different use. This bulb remains much of a mystery to me.
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