Cut-off (but not an absolute) was somewhere between car 600 to 900. Hey, it's a 67 so yes, there ARE exceptions.
I was the original owner of '67 Shelby GT350 #1111 and purchased '67 Shelby GT350 #1688 from the original owner and both cars had functional rear brake scoops. I would assume that the non-functional rear brake scoops came into use later in the production run. As to which cars this happened on first, your guess is as good as mine. shlby66
My car is car #2100 and the rear brake scoops are not functional. Mine is a GT500 but, I don't think that matters on this issue.
My 67 is #1317 and has non-functional scoops as well. Bought the car in L.A. 1977 and still have it in Sweden. Texas Swede
I have #2751 ('67 350) and it has functional rear scoops with ducting and holes in the rear wheel tubs - all original.
I am finishing up my own 67 GT-350 clone. I took the same approach as the previous poster. There is nothing worse than a non-functioning scoop. So I spent the extra $$ and made the lower scoops functional. E-mail me if you would like detailed pictures. david.hofman@easystreet.com Dave