1964-66 frames were different, they had a jackshaft mount on bottom of the frame, and the swingarm mounted low off of it, In 1967 they raised thw swingarm mount and had no provisions whatsoever for a jackshaft, that was the only year for that, In 1968 the frames changed again, top rails, footpegs mounted rearward, jackshaft mounts, etc.... so that seals the deal on it being a 1967.
Also, Gran prix wheels were not used until 1965, the 1964 continental had steel one piece wheels on it.
1964: