Found a list of RPO's as you probably did. I am seeing JH6 is for the lighter duty truck. Other model specific searches say the heavier duty Express is 14,400 lbs. RPO list says meanings can change from year to year.
http://www.crankshaftcoalition.com/wiki/RPO_%28Regular_Production_Option%29_GM_master_list
JH6 11949 P BRK M BRAKE HYD POWER, 4 WHL DISC, 9,900 LBS 940211 E T E A
JH9 11952 P BRK M BRAKE HYD POWER, 4 WHL DISC, 12,300 LBS 990921 E T E A