Bava Batra 161: Signatures of Witnesses on a Document

We learn that when something on a document has been erased, the document is invalid.  The rabbis wonder whether anything can be changed in a document.  Could a word be scratched out and replaced with another word and an explanation about the change, along with the signatures of the witnesses, at the bottom of the document? 

If a person signs on the back of a document, the rabbis are concerned that something could be changed on the inside of the document. Then extra witnesses could sign on the back of the document as well under the guise of creating a strong document with extra witnesses.  

How should witnesses sign a document to best prevent forgeries or additions?  One suggestion is that the first witness signs perpendicular to the text at the bottom of the text on the reverse side of the document.  The writing would move upward toward the top of the document.  In this way there would be no mistake as to any additions to the text.  But documents are valid as long as witnesses sign the document.  Does it matter how they sign; which words they use?

We learn that a number of rabbis would use symbols to sign their names.  Rava would use a picture of a fish.  Rabbi Chanina would draw a palm branch.  Rabbi Chisda would sign by using the letter samech and Rav Hoshiya would sign by using the letter ayin.  Rava bar Rav Huna would draw a ship's mast as his signature.  The rabbis agree that one would sign just his father's name to symbolize his own name.

Finally, Mar Zutra suggests that witnesses sign from the bottom of the text upward, all on one line, ensuring that the text is not later extended beneath those signatures.