They could be. The GN had similar cars then, with the same inside length. Most of the GN's cars had a different bulkhead, closer to the old Con-Cor model, but I imagine that was because they came from a different builder. The GN 161000-161299 series, later BN 618400-618599, built in 1965, were "close enough", as were the BN 621100-621199 series, built in 1970.
So, yes, I would say they could have been in service in 1967, although you'd have to find the exact prototype to be sure. I'm comfortable using two of them on my early 70s BN, and also have one painted GN Big Sky Blue.