See if this helps ... If you look right on the top of the head, about mid-way down on the drivers side, you should find the stamp number. You might need a flashlight to see it, depending on how dirty your engine is. It should be located right where the top of the head and the bottom of the valve cover and everything else meet up. If it wasn't pouring right now and I wasn't sick, I'd go take a pic for you.
On a side note, Everything i've read suggests that the 99's also had this problem. However, I have a 9/99 build date on my XJ and have the 0630 head on the orig. motor. So perhaps any 99's that have the 0331 are extremely late 99 models, right as they were switching over to the 2000 model year
I found under search