I would concur with your assessment. Having had new cars and sold carpet for decades to the Rootes community, I have never seen factory info that specifically defined when cut or loop carpet was used. It is my observation that late ’65 was the period cut pile was generally discontinued and loop pile introduced as the primary. However, in the 65/66 era there were a lot of what I call hybrid cars. That is cars with a mix of interior and exterior early and late traits. Round corner doors with interior vents and early interior pattern and layout with late material. I’ve seen late cars with cut pile.
The point is that no one can argue or prove a specific demarcation point for cut VS loop pile. Although loop was generally used in the MK1A’s, some had cut. So use what you want. I will tell you that most customers preferred cut pile because it has a perceived more luxurious look.