This is a great thread idea, but I think all this SN info is not very useful unless we associate it with specific, identifying attributes. Here are some ideas for future (and previous) posts:
SN / MADE IN USA Stamping - Serial numbers and/or the alpha prefixes may be a key, and for some reason, some frames do not have MADE IN USA stamped in the BB shell.
Bike Size & Head Tube Length - I believe head tubes are shorter on newer years given size on older frames. Not sure what the year change was, but I have read that early models were not suspension corrected.
Head Tube Shape - Some 'Zangs (pre-1996 I believe) have a slightly thicker collar at the top and bottom of the headtube. Later models are straight.
Rear Brake Hanger - Newer models (starting perhaps 1996) have the ring welded to the seattube, earlier models have a welded cable stop.
Rear Dropouts - GT logos, plain, removable, etc. Not even sure how many options there are.
Cable Routing - Someone mentioned internal routing on a very early model. Different types or locations of barrels.
Stickers - Obviously not a good identifier unless there is some reason to believe they are original.
Photo - A simple photo of the frame could help with other attributes that I didn't think of here.
There may be others, but I think it's a good start.
May want to add seat tube insert/shim or not to your list. I think on the head tube machining/collars they just changed it from external in the early years to internal in the later years. I think the head tube lengths have been very consistent on the 25 or so that I've had through my hands in the last couple years.
Other than a half degree change in the head angle from the first couple years to all the later years there has been little or no change in the life of the Xizang in terms of angles, suspension adjustments or TTL's. I've put all year frames post 91 up against my 91 and they're all so close you'd need a good protractor to tell/find any difference at all.
GT's Spoken here. Current projects: 92 GT Team member RTS-1, NOS 94 GT Edge steel viewforum.php?f=49