66Mk1a, the 5A26 means January 26th, 1965, which is the date the head was cast. You will also find casting numbers and they can help a little more as most literature goes by that number. The Jan 65 cast date places your engine in the either catagory (260/289). Check the block casting number, but that is not foolproof either, as this can help because the 260 blocks were usually cast in 1964 or earlier. I do not have my book handy, so I can’t give exact info at this time.