Maalin mishtaros leyuchsin

Please keep davening for Moshe David ben Devorah.

Kesuvos 24 says (abridged): If one wrote 'I, ploni the Kohen, lent money to ...' and witnesses signed on it, can we consider the man a kohen? Do we consider the witnesses to be testifying only about the loan involved, or on everything in the document, including the fact the ploni is a kohen? It's a machlokes Rav Huna and Rav Chisda. One said we can consider him a kohen based on this and one said we may not.

Tosfos s.v. Chad seems to pasken that this is good enough, and in Kiddushin 76b s.v. ein is more explicit. But Rambam (Issurei Biah 20:9) says this is not good enough. Shulchan Aruch 3:2 paskens like Rambam. Rashba also argues on Rambam (like Tosfos), see Beis Shmuel 3:8. A question on the Rambam - why in Halacha 4 does he say maalin mitruma leyuchsin, but in Halacha 9 he departs from the words the Gemara uses and he says maalin mishtaros likehuna, and not leyuchsin? I'm not sure.

Ketzos Hachoshen asks (28:6) why doesn't the Gemara ask "isn't this mipi ksavam? Meaning, testimony is oral. If we consider someone a Kohen because of a signed document, that is written testimony and not oral testimony and should not be accepted? He answers that because all families are bechezkas kashrus, and because one witness is enough in Issurim if not for the fact that the chachamim made yuchsin special and they required two witnesses, so this requirement (to consider them meyuchasim) is only midirabanan, and in this case they accepted written testimony, but he concludes that this question still needs a good answer.

I'm wondering if the yichus is determined from the document itself, or do we use the document to determine what the chazakah of that person was then. If the latter, we are only using the document to see what the chazaka was, not to decide based on this document.

Also, the mishna in Kiddushin 76a says one who was signed as a witness in the registers of Yeshana near Tzipori is considered meyuchas. Rashi seems to say two explanations, but they it is unclear if they are different. He says because they used to record who was meyuchas and only use them to judge. But what does witnesses have to do with judges? Apparently Rashi is very medayek in "kol hakasher leha'id kasher ladun" (see Nida 49b). So they kept a list of kosher eidim not for their use as eidim, but for their use as dayanim. (Because there is no halacha you need a meyuchas to testify). However, Tosfos Yom Tov bring Rashi - and Rashi also says this on the mishna - there was a sanhedrin that investigated people to determine if they were meyuchas. So it wasn't a register of dayanim, it was a register of investigated people. Tosfos Y"T also says didn't have the word "eid" in his mishna. Others discuss if the word Archei is spelled with an ayin, which means register or aleph, which means the elders. Rambam in Peirush Hamishna says if someone was an eid in this register, then they were meyuchas. He mentions nothing about a register due to a beis din investigating their yuchsin.

So it's possible that the register is not what makes us decide he's meyuchas, the register is a record of what was previously decided by a beis din investigating someone's status.