See the thesis ORAL TRADITION AND SCRIBAL CONVENTIONS IN THE DOCUMENTS ATTRIBUTED TO THE PROPHET MUḤAMMAD by Sarah Zubair Mirza, Department of Near Eastern Studies,University of Michigan, 2010.
...
To
Serjeant, their standardized contents and ideology make these letters suspicious.
Though the letters contain conventional phrases found in the Prophet’s letters to the
Arabian tribes, the style seems too “sophisticated,” as if phrases were taken from
documentary material available to the redactors. Serjeant suggests that these letters
were created in the age of the Umayyad Caliph ‘Umar II (99-101/717-20), who is
credited with writing to the princes of Transoxiana, the King of Sindh, and the
Byzantine Emperor Leo III, to submit to Islam.
...
5.1 Palaeographical study of supposed originals of Prophetical documents
As for any surviving material traces of documents written by the Prophet Muhammad,
beginning in the late nineteenth century, a number of “original” Prophetical
documents (mostly on leather) came to light. These all fall within the tradition of the
Prophet’s letters sent to foreign rulers, and their texts correspond with the redactions
in literary transmission. Four of the leather Prophetical manuscripts (letters addressed
to al-Muqawqas, al-Mundhir b. Sawā, al-Ḥārith b. al-Ghassānī, and the “false prophet”
Musaylama) are now housed in the Pavilion of the Sacred Relics, in Topkapı Sarayı,
Istanbul, where they are not on permanent exhibition, and 2004 saw the first
publication of images of them by manager Hilmi Aydin, while the remaining
documents, reported in Arabic newspapers at the time of their discovery, have now
disappeared from view.
This chapter will present a palaeographical analysis based on published images of seven
of the discovered manuscripts, which will be presented in their order of discovery.
These include the letters addressed to: 1) al-Muqawqas, 2) al-Mundhir b. Sawā, 3) al-
Najāshī, 4) Hiraql, 5) Kisrā, 6) the sons of Julandā, and 7) al-Ḥārith b. al-Ghassānī.
...
5.1.1 Proselytizing letter to al-Muqawqas
The letter to al-Muqawqas of Egypt was found in 1850 by French Egyptologist Étienne
Barthélemy in a monastery at Akhīm in Upper Egypt. It was first published along with
a letter from C. Belin dated Oct. 3, 1852 in the Journal Asiatique in 1854.
...
K. Öhrnberg in the second edition of
the Encyclopedia of Islam states that the Prophet’s embassy to al-Muqawqas is considered
legendary, and that the leather letter was recognized as inauthentic based on historical
and paleographic considerations.
...
In the modern age the Prophetical documents have had limited success as relics. The
manuscripts that have surfaced were dismissed early on by Western scholars as
forgeries based on historical considerations and palaeographical grounds, while the
current locations of the manuscripts in the Islamic world are little-known.
It's not easy to summarize this over 300 page thesis, let alone scratch the surface of the 700 references. But clearly this is the place to start if you want to seriously study the letters.
There is no indication that any of the supposedly-original letters of the Prophet, all "found" in the last 200 years, are actually physical original letters.
