Bava Batra 161 - Preventing Forgery in a "Bound" Document

In a regular document the witnesses sign immediately at the end of the document's text, thus precluding the possibility of tampering with it.

In contrast, a "bound" document requires that the witnesses sign between each fold. However, this opens a possibility of forgery.

Should we be concerned that, having the witnesses already signed above, he will write an additional clause at the end of the document, beneficial to him? - No, because words "firm and established" are written at the end of a bound document.

What if he erases the words "firm and established," adds to the document, then writes the words "firm and established" again? -  Additions do not invalidate the document, but erasures, even certified with "firm and established", do invalidate it.

Art: John Turing - Still Life with Documents