[Healeys] cylinder head questions

[warthodson at aol.com]

A friend purchased a Healey 6 cylinder head at a swap meet recently. It has
AEC721 cast into it. My BJ8 head has AEC960 cast into it. Interestingly, the
Denis Welch aluminum cylinder head has AEC721 cast into it. How many different
6 cylinder heads were fitted to the big Healeys & what were the differences
between them?  Is there any reason you know for Denis Welch using the AEC721
for the basis of their aluminum head? I did notice that the boss for the water
temperature sensor is much more accessible (I.E. getting a wrench on the nut)
on the AEC721 vs. the AEC960 which raises the question why did the factory
change the design?
Gary Hodson