My 5th and 6th numbers are 91 and the last digit is 2. Rest are letters. Unless they did it at the end (week 2) I don't think they are indicating that now. Unless my iPad has been sitting around for 91 weeks. What are yours?
Odds are they just switched to letters & numbers because they can hold 36 different characters in each position instead of just ten when using alphabetical only. I am sure the serials still represent dates made etc.