The later valve bodies extend farther back over the case; towards the low/reverse servo, whereas the earlier valve bodies (some of them) do not extend back far enough to cover orifices in the later cases. I'm not saying that the later vb's (71-up)can automatically be used on 66-up cases; I don't know if he does any mods to allow that. But I've seen earlier vb's have the extension go farther back as well.
