There have been many changes to the P226 since it came out. Many, many, many changes. The most noticeable one was when they went to the stainless steel slide. The previous design was sheet steel with a machined breech block.
version#1 was the XM9 prototype, had straight rails, matt sights
version#2 was the first commercial gun, frames cracked all the time
version#3 had scalloped cuts in the rails, frames cracked all the time
version#4 was the first one where the frame didn't crack, around U150000
version#5 was essentially the model they sold up until the stainless steel slide came out (and sometime after in fact, they stopped making them in 2008), this one is recognizable because they rounded off the edges of the slide. Various alterations were made, they removed the grooves from the trigger, the breechblock was altered around U550000 to make it more fully fill the slide to stop the breechblock pin from cracking, also the trigger bar spring was altered to a wound type to stop it from breaking and "eating" through the frame behind the mag release. The mainspring was also shortened and put into a plastic holder to make it easier to remove. The grips were changed several times, the ones around 1996 were the worst, too slippery.
version#6 is the one with the stainless slide, which first came out in 1996.
There have been many variations since, but they're mechanically the same as version #6, except for optional things like the SRT. The X-5 is the only substantially different P226 model since then.
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