Buffet serial numbers are typically stamped on the back side of the upper and lower joints (the two middle sections with keys) directly above the tenon (the piece where the cork is that connects the joints). Although there are two numbers, the numbers should match.
The serial numbers on a Buffet clarinet may vary depending on the model, but if a Buffet clarinet has a stamp, they are always in the same spot on the top and bottom joints. If the clarinet does not have a number in that location, then it may be possible that the number has either worn off or that it is an R13 model. Buffet did not always stamp the R13 models.