I just bought a nice 3913 over the weekend that came with the 2 magazines pictured. I would like to add a few more original magazine. I will be keeping an eye out at my LGS and gun shows. How do I tell a 3913 magazine from something else that won't won't be the correct fit? Mine do not have any markings other that S&W stamps, so not sure if it's a measurement or something else that will tell me what I'm looking for. Thanks in advance for any help you guys can provide!