Are all the current production Glock mags Gen 4 compatible for lefties i.e. dual notches? Being a lefty and having the capability to comfortably operate the mag release with the thumb instead of the middle finger is nice, but the guessing game of ordering mags online can be frustrating since most do not specify whether its Gen 4 compatible. I would assume that all the new mags being shipped to stores now have dual notches but some sites like Cheaper Than Dirt have some mags that they specify are NOT Gen 4 compatible. Is there any way to tell from the product number? I see some that are prefixed with GLG and some with MF but who knows..