There is an earlier thread on the TR6 head differences. Some one out there should refer you to it. I don't know how yet.
However, the 1st thing to do, when it comes to the TR6 head, is to stop thinking "early and late" and start thinking narrow intake ports and wide intake ports. The exhaust ports are the same on all. You can't even think low compression or high compression any longer to differentiate early and late. For there are low and high compression heads in both narrow and wide intake port configurations.
Now, with that said, the later year head has the wide sweeping intake runners. Looking at the manifold gasket numbers, there is one for years 69-71 and one for years 72-76.
All TR6 heads are interchangeable onto any TR6 block, groove or no groove. All exhaust manifolds fit all heads. It is the intake manifold that has to be programmed.