So it looks like the frame stamping on the head-stock started in 1962 until the crossover year in 1965. In 1966 there was a new rounded pad next to the left-hand front engine mount and in 1967 this changed again by using a flat face pad in the same location.
Indeed, 65 models were still on the headstock, 66 was on the front lug but 67 had the raised pad as highlighted in Quintens post.
The open/closed loop frames were both in use at the same time at one point. Like the longer and shorter swing arms, the open loop frames were known as the sports frames, lightnings had this frame for example as did other variants in the sports class. Later the closed loop frame stopped appearing in production, along with it ceased the “late A10 style” front and rear brake with cross over shaft, the huge side covers and as far as I know the dropped yoke setup stopped being part of production except on the clubman models (where they were likely using parts up), as went the deep valance mudguards and everything which resembled the early Star and (Thunderbolt Rocket or) Rocket modes (model name depending on where on the globe you are).