I have one of those CHP revolvers. It is not overstamped. It is indeed a 6" SS model 68. The box it came in is marked "special". It has the red ramp but no white outline rear. It has hogue grips but the box says SB under "stock". Is that the designation for target grips or is "stock" on the box for something else?
The CHP did tests that indicated to them (so they said) that the +P+ .38 was just as good as the .357 so they went with the .38. I think it was because there were several women and smaller stature men coming onto the force at that time and it was easier for them to control. If you did not qualify, you did not graduate from the academy. There was a lot of time and money invested to get them trained thus far and they wanted to have the most people qualify. It was also thought that all members carrying .38 would simplify ammo purchase and using others ammo in an emergency. The CHP has a lot of officers and the amount of ammo they use every month is a big investment.
Of course now it is all changed, to .40 automatics.