Whew! Ya gotta be fast to get in here before carguychris
, but it's all folly anyhow because he can do a much better job at it. But hey, my post will bump it so it's easier for him to find it!
All the four-digit semi-auto pistols are 3rd Gen pistols. There are a couple 3-digit pistols that are also 3rd Gens, they were the "Value line" guns that S&W marketed at a lower price. They were very plain...value kinda pistols. The 908 and 915 are examples, they're a 3-digit model number but surely are 3rd Gen pistols.
The 2nd Gen guns typically can be spotted by the raised rib on the left side of the frame by the top rear of the grip area. I've not had the pleasure of taking apart a 2nd Gen, so I can only assume
that the visible part has something to do with the safety system or hammer/spring of the pistol.
All 5904 and 5906 pistols are 3rd Gens. Same for the 4506 and it's cousins, and the 4006 and it's brethren and the 10mm pistols of course (my favorites), the 1006 and the other 10xx pistols.
As for the square-ish trigger guard, the early 3rd Gens had a more squared front contour and it was rounded out a bit later in the series. It's subtle, but noticeable.