From Fall/Winter 2003, every Balenciaga bag tag carries an uppercase letter that dates it to a half-year season. The system runs backwards through the alphabet, and because it has now cycled, most letters mean two possible seasons. Enter the letter and what you can see on the tag; the decoder returns the candidates and what separates them.
The letter dates the bag; it authenticates nothing, and counterfeits carry plausible letters. Production before Fall/Winter 2003, the runway Lariat, the earliest Firsts and Cities, carries no letter at all, so a letterless tag on claimed early production is correct rather than suspicious, and dating those bags falls to hardware and tag fonts, which their model files cover. The alphabet has cycled: a D is Fall 2003 or Spring 2016, and the tag's own features, plate or leather, silver stamp, back-of-tag letter, print size, decide which reading is honest. The letters E through J and X have only ever been used once, so they date without ambiguity. The style number on the tag front must match the tag back on any genuine bag, and the remaining digits are internal codes that date nothing.