That casting number is used with that bigger valve twin carb head up to 1970. And on Spitfires, except the '68 Spitfire which had a specially ported head to match the earlier 68-701 Spitfire heads on 10 or 10.5-1 engines with GP2s with its 32 Concentrics
and 9-1 compression. The rear head bolts were enlarged probably around '67. And std ports vary quite a bit to look at.
The Spitfires heads may have had a bit of attention. Generally the 68-701 flows around 109cfm, a Thunderbolt minus the manifold around 107 or 108 and they are 27mm. It's not hard to make them much better. They do not have to be bigger in the port, but a 30mm Concentric
would not flow like a GP2
. And going bigger can flow lots more. Disproportionally is actually what gives best results. Because gas speed is increased and there is no loss at lower rpm.
If you are using GP2s a good flowing head would be interesting. Racers used GP2s very successfully for quite a while.