Witnesses on a doc is an argument between Rav Shach and the Chazon Ish--if they are considered to have made their testimony before or after the plea.
I mean to say that Rav Shach is trying to prove that the obligation of an oath in the case of a woman says to her ex husband that he owes 100 zuzim. He says he paid fifty.--that case he is obligated an oath. Rav Shach says this is a case here there is no document [Ketubah], for if there was, he would not be required an oath since the admission in half happened in such a way that he could not deny and also there would be witnesses on the doc their testimony would be considered as coming before the claims. Thus also he would be not obligated in taking an oath. The Chazon Ish holds in this last case witnesses are thought to be coming after the plea so it is like admission in half and he would be obligated to take an oath.
On the way back from the sea I was thinking about this and it seems to me clear. The Chazon Ish is thinking about witnesses. If you have two witnesses that come to court and say he is obligated in half the sum that is demanded he is required to take an oath.--even if they say that they already testified to this ion a previous occasion. And that is true. But Rav Shach's point is witnesses on the document is not the same thing. They only signed it once and that was before the plaintiff came to make his plea.
Thus the only case that the law in the Rambam in laws of marriage 16 is possible is in a place where it is not the custom to write a ketubah.
