http://forum.purseblog.com/gucci/gucci-101-how-to-read-gucci-tags-684537.html#post19030297
This was a very helpful post that may have been missed by some readers of this thread.
As well, to echo others, in my experience the layout and format of the numbers is mostly dependent upon the age. By now Gucci is a pretty well-organized, streamlined mega-brand and so pretty much if you bought anything after the mid-2000's it'll be pretty consistent. Earlier bags, though, varied a lot by season in how the numbers were arranged. Before Tom Ford came along in the 1990's, Gucci was a huge mess and good luck figuring out consistency over time with the codes. Even during his tenure (and the "big" years 1995-2002) they played around with various formats, ranging from XXX-XXXX on top to four, six, or five numbers on the bottom.
I also think, based on various bags I've had/seen, that they changed around whether codes were unique, noted the season, or simply noted the factory where it was made (I've seen several auth bags, for example, from the mid 1990's that have the same codes along the bottom, which I suspect must simply be the location/factory). This is just speculation, but I'd also be curious if some older codes may depend on whether bags were made for the outlets (which also are recognizable for the circular "G" stamp on the tab beside the R-Gucci made in Italy stamp).
In short, don't panic if your bag doesn't necessarily have the exact same marks... but learn the signs. It's much easier to tell if a bag is authentic based on the fonts, hardware, snaps, stitching, etc. than just from a number, especially if the bag is more than a few years old. But, if the bag is current, I'd say anything fishy probably doesn't speak well for the authenticity.