I am doing my second 69, and it seems the tail lite buckets on my last car differ from the previous. My previous car had white paint on the inside, these do not. Both cars are Dearborn cars.
I have never seen white paint used on the inside of a taillight on a 69 or any mustang. I have taken many 69 taillights apart and only seen aluminum spray paint but I am sure their could be exceptions, but definitely not typical.