I’m not sure what the purpose of the cylinder between the engine’s heater inlet and outlets?

https://spokane.craigslist.org/cto/d/spokane-willys-cj-3a-jeep/7487793756.html

“Selling my Willys CJ-3A. Ran when parked 10 years ago. Would need some tinkering but should run. Last licensed in California in 2002. Bill of sale only. $2750 … Trades that have a higher value than my asking price would be considered.
This Would make an awesome father son restoration project.
Muscle Car Rare Patina Classic”