Also known as heat control valve for 914-6. These were originally gray. Inside are springs and what I speculate are asbestos based fiber discs for sealing the openings.

These were superseded to a single part number (022-256-206) associated with 914-4. The key differences being the 914-4 are painted black, and a sizable hole in the end cap is missing.

Right and left are distinguished by having the actuating arm on opposite sides.

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Photo credit Retro Automotive Products eBay listing.