Legally the badge remained the property of the AA so should be returned if you ceased to be a member. The Serial number on the badge allowed the AA to keep track of who it had been issued to. It would not give any information about the vehical, and I doubt the AA have any record left of badge numbers anyway. These badges were replaced by unnumbered ones in the late 60s. I had a friend who's first job on leaving school was sending out new badges to those members who returned the old ones, which he just threw in a bin to go for scrap. He was nearly in tears 15 years later when we walked round Bueulie(?) Autojumble and saw how much they were being sold for!