Hello Roger,
Here are the pictures I took of both the cylinder head and the engine block. I think things may become a little clearer now. Once I discovered the part number C 6733-1 cast on the head, I rushed to my X120 parts manual. There is a C 6733 cylinder head listed for the XK120, but no C 6733-1, which leads me to think that the head on my car could possibly be also from an XK140. I don’t own an XK140 parts manual so I cannot establish that fact for sure. So, thank you for any light you may be able to shed on the matter.