I, too, have a BB car 12B Dec 68. It came with a UY, with IO and OO tanks, like it should have. My post on this car shows a pic of the radiator. It has curved neck. It had the original core until I replaced it 2 years ago using the original top and bottom plates. One thing that I question is that it has a J date =Sept. Not sure why it is so early except it was probably used because of a shortage as CRG report states that substitutes were used, I will add, usually the next bigger radiator substituted, as I understand.
I also have another radiator I acquired recently and saw the car it came from. It is a 12C Dec 68 BB Camaro. it still has the tag=ZD with correct tanks, original core with OO & IL tanks with a date of M=Dec.
Steve, your BB radiator sounds like an real oddity and perhaps a victim of the same shortage at the same time. I really don't know, but I guess anything is possible. Both of my radiators are manual trans. type also.I'll add, all cars referred to are Norwood cars as I live close to Nor. Plant.