I don't think all the 27s were marked CHP. I had one in the 1970s that I carried a 6" 25-2 in. A big difference between the 27 and the 2800 is the way the holster covers the rear of the trigger guard. The rear seam on the 27 goes all the way around the trigger guard till it bumps up against the grips. This kind of interferes with getting your middle finger all the way up against the grip especially when using target grips. The 2800 leaves a small portion of the bottom of the trigger guard exposed and covers the rear of the trigger guard with part of the holster that acts sort of like a strap. It makes it much better for getting a good grip on the revolver during the draw. The 27 was the first holster I know of that had a "jacket slot". The belt loop was attached to the holster near the muzzle end so your jacket could go between the holster and the belt.
The original idea of the Berns-Martin was a high ride holster (to keep the gun out of the snow) that you didn't have to pull a long barreled revolver so high to get it out. In law enforcement, this meant a four or six inch barrel revolver could be high enough on the belt to clear the car seat plus it was difficult for a suspect to snatch especially from the rear.
I still have a Speed Safety holster for a 4 inch N frame which was similar. I don't use it on duty because we wear nylon gear now. I also have a break front holster for a 1911 that is not an Auto Draw model 3000. I'll have to find it an see who made it an what it is called.