I haven't had or even looked at a 4.2 in over 10 years and my last one was in a cj but just a guess... unless it's a molded hose, usually that sort of hose is only identified by inside diameter and length.... IE 5' roll of 3/4" coolant (or heater) hose.
I don't mean to be disrespectful, but I've read the question several times and I'm not sure understand it.
The thermostat is housed within the engine assembly and has no hoses connected to it.
The thermostat housing encloses the thermostat and has two hoses attached to it.
The smaller of the two goes from the thermostat housing to the intake manifold and is called, as Jar06896 states, "heater hose, thermostat housing to intake manifold". The part number varies depending on the year and the engine size. It's probably available in bulk if you don't care if it matches the exact shape of the factory hose. Take the old one to the auto parts store so they can match it up for size and length. Check your hose clamps and buy new ones if there is any question about usability. You'll probably need some anti-freeze too.
The larger of the two hoses is called "upper radiator hose". Same thing as heater hose applies when purchasing a new one (check clamps and anti-freeze). When replacing an upper hose, it's smart to replace the lower hose too.
If this and the above posts don't provide an answer, post a pic of the part in question.
I just read your earlier post, I see you have an '89 4.2.
The old part number is 5200 3908. It probably updates to a different part number. Your Jeep parts person can give you the new part number and tell you if it's still available.
I believe the one from the heater has two different sized openings and is probably a Jeep part, rather than available aftermarket. It'd be worthwhile to check aftermarket suppliers though (Quadratec or Morris 4X4 or Rock Auto).
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