UPDATE 1/5/2019: We have identified the NSN of this tube. For our application (an early, low serial number HMMWV), the correct NSN is 4720-01-473-2352 [4720014732352]. The manufacturer number (does not appear to be an AM General number) is RCSK18121.
Later models (SERIAL NUMBER 196901 AND ABOVE) use a different tube, 4710-01-488-6076 [4710014886076], which carries AM General P/N 4710014886076.
Currently using heater hose on this location and trying to locate either NSN or part number for this tubing.