A good way (but definitely not the most efficient way) to check your XBOX version is to do a pin header install, then check your LFRAME pin (it's across from the Ground pin, or pin #2, diagnol) - if you don't get a signal/voltage reading from that pin (on 1.0/1.1/1.2 XBOXes you'll get 3.3V from the LFRAME pin), you have a v1.3 (as I discovered yesterday when attempting to mod my friend's XBOX).
So, I had to order up a new Xecuter 2.2 Lite chip for him (works with 1.3). I guess the new modchips "trick" the XBOX into thinking that D0 is enabled so that LPC mods once again function normally, without the need for an LFRAME signal. Interesting stuff...
The SN guideline works well, too. If your SN is xxxxx 31xxx, you definitely have a 1.3. If it was manufactured AFTER April, chances are it's a 1.3 model.