Here just a summary of my findings:
- After checking my "usual suspects" among the books of tafsir I didn't find anybody claiming that verse 61:6 had any specific reading of the sahabah.
- I consulted my Manahil al-'Irfan of az-Zurqani with the same result. I'd expect al-Itqan of as-Suyuti to have more information available, but neither have time nor by now have the expectation that this will be more fruitful.
- A search on the internet brought me to the book of ibn abi Dawod أبو بكر بن أبي داود called Kitab al-Mashif كتاب المصاحف which offers a few specific readings of the sahabah and their moshafs. And in the same article I found your quote which was referring to Arthur Jeffrey. However at my first reading it was unclear to me whether Jeffrey wrote something in his introduction of the book of ibn abi Dawod or only in his own book (thesis): "Materials for the history of the text of the Qur'an: the old codices" where I found it in page 170.
In page 116 however Jeffrey quotes a source of information which is Qurrat 'ayn al-Qurra' قرة عين القراء في القراءات of a rather unknown author called Ibrahim al-Marandi بديع الإسلام أبي إسحاق إبراهيم بن محمد بن علي القواسي المَرَنْدي الأذربيجاني المقرئ الحنفي who finished his work 588a.H. (Marand)
of which a handwritten copy of the manuscript exists at the "El Escorial library in Madrid" and -apparently- another in Istanbul in the "Waly ad-Dyn Afandi Library". Both have around 225/226 pages which are hardly readable (my impression is that he wanted to write about the 7 qira'at this is what he claimed in the introduction this would go ahead with book al-Hamadhani quoted later). Note that Nöldeke already mentioned this book in his "Geschichte des Qorâns"(history of the Qu'ran) and so did others afterwards, but he didn't mention a deviation of Ubay's reading in surah 61 at all.
This book was last copied in 747 a.H.. The author apparently only had one teacher: Abu Ya'aqub Yusuf ibn Musa al-Hanafi al-Marandi أبو يعقوب يوسف بن موسى الحنفي المرندي (no biography available) -the author didn't quote any other- in this reported from three teachers. Here a quote from an effort of an other anonymous person form an internet forum, I've added my findings in the text:
In the following I'm translating from Arabic language, as these translations are of my own take them carefully
2 - شيوخه: لم يذكر المؤلف إلا شيخا واحدا من شيوخه وهو:أبو يعقوب يوسف بن موسى الحنفي المرندي (وكذلك لم أجد إلى الآن ترجمة له بعد البحث)
2 - His sheikhs: The author only mentioned one of his sheikhs, namely: Abu Ya'qub Yusuf ibn Musa al-Hanafi al-Marandi (and I have not found his biography until now after researching).
وهذا الشيخ كما هو في أسانيده يروي مباشرة عن الإمام أبي العلاء الهمذاني صاحب "غاية الاختصار" وعن الشيخ أبي الأزهر المظفر بن أبي القاسم بن عبد الله الصيدلاني، (وأيضا لم أجد له ترجمة إلى الآن بعد بحث قليل)
This sheikh, in his chain of narrators, narrates directly on the authority of Imam abi Al-Alaa' Al-Hamdhani (Born 488 a.H.; died 565 a.H. according ad-Dahahbi) , the author of "Ghayat al-Ikhtissar"(Book on the 10 qira'at not the shafi'i fiqh book of abu Shuja') and on the authority of Sheikh Abi Al-Azhar Al-Mudhafar Bin Abi Al-Qasim Bin Abdullah Al-Saydlani.(and I have not found his biography until now after a short search) (According the link added to the Arabic text he died 569 a.H. according imam a-Dhahabi) .
،وعن الشيخ: أبي الحسن علي بن عساكر البطائحي النحوي البغدادي،وهذا شيخ مشهور معروف لكن لم أجد التلميذ المذكور ضمن تلاميذه فهذا يعتبر "استدراكا " في هذا الموضوع
And on the authority of the sheikh: Abi Al-Hasan Ali bin 'Asaker Al-Bataihi Al-Nahawi Al-Baghdadi(Born 443a.H. died 520 a.H. according a-Dhahabi), and this is a well-known famous sheikh, but I did not find the aforementioned student among his students. So this would be a "complement".
والمرندي رحمه الله (يروي) كل المعلومات في كتابه هذا بالسند عن هذا الشيخ عن المشايخ الثلاثة المذكورين سواء المرويات المتعلقة بالقراءات أو المتعلقة بالآثار
Al-Marandi, may God have mercy on him, (narrates) all the information in hiss book with the chain of transmission on the authority of this sheikh via the three mentioned Sheikhs, whether the narrations related to the readings (qira'at) or related to Athar.
This actually seems the most promising source.
None of the following books on shadh qira'at quoted it:
- Ibn Janni's ابن جني (died 392 a.H.) al-Muhtassab المحتسب في تبيين وجوه شواذ القراءات والإيضاح عنها
- Ibn Khalawayh's ابن خالويه (died 370 a.H.) Mukhrassar fi shawadh al-Qur'an مختصر في شواذ القرآن من كتاب البديع
- Abu al-Baqa'a al-'Okbari's أبو البقاء العكبري (died 616 a.H.) I'rab al-QIra'at a-Shadah إعراب القراءات الشاذة.
- Al-Karmani رضي الدين الكرماني (died 563 a.H.) Shwadah al-Qur'an شواذ القراءات
Sadly many ancient books of the 3rd-5th hijri centrury are either not yet printed or counted as lost. Among the the boook of ibn Ashtah ابن اشته on which as-Suyuti relies a lot in his al-Itqan.