Took a few pictures of an old Glock magazine along side a newer model. There are a few easy ways to tell them apart. The easiest is the notch at the top. On older (non drop free) magazines the notch is a "U" shape, while the newer (Drop Free) style have a squared off notch
From the side and bottom more differences are able to be seen.
From both sides the obvious difference is the three dots down the sides
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