Okay Burb Gurus !!!!

[Already Gone]

As most of you know I'm hunting for a Burb and am still looking. My question is why is it that on some 69 Burbs the factory A/C vents come out of the dash as do the 72's and rear air is mounted at the back just inside the rear doors/tailgate and some have that roof mounted thing that goes from front to back. Now I always thought the roof mount was a dealer installed item but a guy sends me spid pic and the 69 Burb has roof air and the code C69 is on the spid. The only thing I can think of is that there might have been a mid-year change-up in the factory A/C installs. Any one ?????????