I don't understand why the gemara wants to use the fact that afar/efer is not on r' Yishmaels list as proof that EFER cannot be used for Sotah.

In the three cases listed the pasuk is specific and the halacha lmosshe misinai(HLM) adds to what the pasuk says can or cannnot be used. ie. for kisuy hadam the pasuk says you cover in dirt and the HLM adds that you can cover blood with other items as well. For the nazir - the pasuk says he cant shave with a t'aar and the HLM adds that he cant remove hair in any way. For gittin the pasuk says that a get has to be written as a sefer on klaf and the HLM comes to add other items that are acceptable for a get to be written on. In all three cases the additions given HLM are entirely different and new from what the pasuk says and are not called or referred to as the item written in the pasuk.

Our whole Kasha about whether EFER can be used for Sotah is based on Bais hillels opinion that EFER is sometimes called AFAR meaning that they are the same thing. if that is the case there would be no need for HLM to come in and say that EFER would be ok for Sotah because it would not be adding anything or telling me something that I did not already know.

I can understand the question of wanting to know whether EFER can be used for Sotah but its lack of inclusion on this list doesn't prove anything one way or the other.

Thank you.

Ari, New Jersey