Ok, I think I have it figured out. From what I can tell, it appears that the serial number might actually be 21 QK-H 1191. It's hard to see, but I could just make out the hint of a "tail" on the "O", which may make it a "Q" instead. Being a "Q" also makes it work out with the listed model codes, with a "QK" being a 4400 1.5 ton 161" wheelbase. The rest works out like this; 21 is the assembly plant, Janesville Wisconsin, H is the month built, June, and the rest is the sequential number.
