Correction:
区 "Ku" should be translated as "ward" in English in my previous post.
Umayakami-mura is a misreading of Mayakami-mura. The first kanji 馬 can be read "uma". Also, Oyakamaken is obviously a typo of Okayama-ken.
岡山県御津郡馬屋上村大字三和 Ōaza(= larger section of village) Miwa, Mayakami-mura, Mitsu-gun(= county), Okayama-ken(= prefecture) is the present 岡山県岡山市北区三和 Miwa, Kita-ku, Okayama-shi, Okayama-ken.
Google マップで地図を検索。乗換案内、路線図、ドライブルート、ストリートビューも。見やすい地図でお店やサービス、地域の情報を検索できます。世界地図も日本語で、旅のプランにも便利。
Yeitaro and Eitaro are just variations in romaji transliteration. Those two names refer to the same person.
You know his detailed address, so it could be possible to reach them even with romaji names. 岡山市北区役所 Okayama-shi Kita-ku Word Office might have their information.
You can contact them via e-mail.
(all in Japanese)
I remember that there was a member who wanted to get her family's records(koseki) like you.
She sent a Japanese e-mail to Obama city hall, as in the thread linked above, and got a reply in Japanese. She PMed me their reply, and the city hall said that the followings were necessary to get koseki in the e-mail.
- a document to confirm her personal identification (official ones with face photograph such like passport or residence card)
- an official document to confirm the relation of consanguinity between her and her great-great-grandparents, and the Japanese translation of it
They also asked her to contact 小浜市役所市民課 Obama City Hall Community Services Division via telephone through someone who can speak Japanese before she went to the city hall in order to check about the necessary documents and whether it's possible to give koseki or not.
I don't know whether she succeeded in obtaining koseki or not since she didn't come back to the forum, but you would need similar documents. I recommend contacting the ward office first, anyway. (Okayama-shi is a relatively big city. There must be someone who can speak English there, so I think it's OK to send an English e-mail. Please tell us if you don't get their reply. We might be able to help you with translating your e-mail into Japanese.)