As stated, you are going to have to call Glock for more info. I have checked as well and can't seem to find any 21's with the YA prefix. The best I can find is that early generation G22's ended at YA. Not sure what came after that.
When you do find info, would you mind posting it here:
http://glocktalk.com/forums/showthread.php?t=1287557
Its not everyday a strange and unknown serial number pops up for GT.
Edit: Im almost willing to bet that its a special run for a PD or something. You may have a rare mode.