When I bought a 1967 Sports (Herald) 1200 convertible some 10 years ago, it somehow had been registered in New York as a 1964 model. How that could have happened, I don't know, but NY's DMV accepted the data on the BMIHT certificate as proof that a May 1967-built car was, indeed, a 1967 model!
That said, I suggest leaving the paperwork as-is. If nothing else, it likely tells the history of a car that just plain didn't sell for the better part of two years, or perhaps one that a dealer held onto for other reasons (demonstrator? personal use?), after which time the car was finally sold and first titled as a 1967. Back then, one could easily get away with that. Even manufacturers like Triumph could do that legally. By 1968, when Federal safety and emissions standards came into play, model year / year of manufacture dominated.