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The G27 has had three different frame types, usually designated as Gen2.5, Gen3, and Gen4. Here is a photo showing the different frames, with the generations labeled; this should enable you to figure out which one you have:
The only noticeable difference between Gen2.5 and Gen3 is the fingergrooves on the frame. In the Gen2.5, the front of the fingergrooves are smooth (no checkering); in the Gen3, Glock added several rows of checkering in each fingergroove.
"That means that no matter how we reform health care, we will keep this promise to the American people: If you like your doctor, you will be able to keep your doctor, period. If you like your health care plan, you'll be able to keep your health care plan, period. No one will take it away, no matter what." – President Obama, AMA speech, June 15, 2009.
see 13:20 - 13:58