Missing after (a) is (TOSFOS fisrt explanation)
8)[line 2] LO SHE'AMAR REBBI AKIVA SHEL SID DEIHAH HEIMENAH ELA SHEL TZEMER DEIHAH HEIMENAH- (lit. Rebbi Akiva did not say that [the Pasuch mark] of plaster is weaker than it, rather, he said that [the Pasuch mark] of wool is weaker than it.) (a) Rebbi Akiva only listed plaster after snow as a response to Rebbi Yishmael (see above, entry #4); (b) Rebbi Nasan intends to change the Girsa in the Mishnah in the opinion of Rebbi Yishmael, also, that wool, and not plaster, is listed after snow (TOSFOS, 2nd explanation). In either case, Rebbi Akiva rules that white wool is the mark second in whiteness to snow, and that all colors mix with each other, and that they are not grouped into Avos and Toldos.
Looking more properly at Tosfos 6b, DH v'Im, and at what we wrote in "Background to the Daf," I saw that "(a) Rabbi Akiva only listed plaster after snow as a response to Rebbi Yishmael" is in fact identical to the first explanation of Tosfos: "Kelomar Lo Nakat Sid Davka Ela Kai A'Tana Kama" -- "that is to say, Rebbi Akiva did not use the word 'Sid' specifically but only said 'Sid' because he is responding to what the Tana Kama said."
Tosfos is referring to the end of Nega'im 1:2 where Rebbi Akiva says "plaster is weaker than it." The first explanation of Tosfos is that Rebbu Akiva did not mean specifically "plaster"; he said "plaster" only because he was responding to what the Tana Kama (who is Rebbi Yishmael) said ("plaster"), but the opinion of Rebbi Akiva himself is that wool is weaker than it.
This is what we referred to in 6b, entry 8: "see above, entry #4," where in 6a, entry 4, we wrote "Rebbi Akiva is responding to the words of Rebbi Yishmael."
Dovid Bloom