Head saver shims or gaskets.

[Geoff Kingston]

Brian confirmed what I suspected regarding the early head, it needs a fair bit of machining work on it to make it serviceable, he suggested in fact that the whole face could have been welded and then cut back, as it stands the differences in the shape and size of the combustion chambers plus the way a few of the valve seats had been cut would not have given a smooth running engine.
The later head by comparison needs minimal work so that is the one that will go on the rebuilt block and the early head will receive further work and be kept as a spare.
As an aside he also told me that the early BSA motorcycle heads suffered similar corrosion problems to the early Bristol cylinder heads and presented similar challenges in repair and machining, at least it is in the right hands and will be saved rather than scrapped.
GeoffK