In the Mustang and Ford Small Block V8 book by Mr. Mannel there are references made to changes L10 and L11 on the 289 non HiPo head in 65 but the changes were not incorporated in the HiPo head till 66.
I have a set of heads, C5OE, F52 that appear to have those changes incorporated. I have seen examples of just the guide slots being machined, but I believe the changes on these heads are factory changes because of the additional circumference change to allow use of the smaller valve spring and the valve guide casting configuration is the later form.
Has there been cases of this early incorpaoration?