Good point. I did order an SF, but the order form was dated 2009(?), so it pre-dated the 21 Gen 4. On the order form I had the choice between 21 and 21 SF. I guess it'll be a surprise. Honestly, I really don't care one way or the other, just curious. I trust both of them to exceed my expectations...If you ordered a G21 SF, it seems as though you would get a Gen 3. The Gen 4 does not come in an SF designated model as the back straps are interchangeable. That's just a guess though.