SIMP, the change is only cosmetic and it isn't a big difference. I don't know exactly when the change occurred, but I believe it's been in the last six months or less. The older models have a horizontal mill cut at the bottom of the slide just above the frame where the frame narrows above the front of the trigger guard and runs to the front of the slide. The new SR9s omit the mill cut in favor of the vertical mill cut on the front of the slide like the SR40 and now the SR45. So, now the slides on all 3 full size pistols have a similar appearance.
