The reason why the Islamic patriarchs elected not to unveil the face of Muhammed had to do with the historical polemic between the Dyophysites and Monophysites about the nature of Jesus the Christ.
The Dyophysite Christians from the Dyophysite Church of Hagia Sophia (Western Rome) believed that Jesus the Christ, like Serapis, had two natures (half man and half god). However, the Monophysite Christians from the Monophysite Church of Constantinople (Eastern Rome) believed that Jesus, like Osir, only had one divine nature – they asserted that in the person of Jesus the Christ, there was only one, divine nature rather than two natures (divine and human) as asserted by the Dyophysites at the Council of Chalcedon.
The Nestorian Christians buttressed the Monophysite argument, in part, by stating that “it’s impossible for a daughter of man to give birth to the son of god when god is obviously incorporeal (invisible) – they asserted that the humanity of Jesus was a separate substance from the divinity of god, which is reminiscent of the separation between Heru (the sun) and Osir (the mythological father).
Arius, the powerful African bishop, weakened the Dyophysite argument in part with his Arian formulation that’s directed totally against the Christological doctrine about the dual natures of Jesus. Prior to Arius, some religious thinkers had denied the humanity of Christ and some his divinity, but Arius was the first to deny both. Arius argued that the son was a creature, who’s dissimilar from the father (Osir).
Around 7th-8th Century CE, the Nestorian Christians left Constantinople and its environs and migrated to present day Iran, where they converted the Iranian/Sassanid aristocrats to Nestorian Christianity. The Nestorian Christians and Monophysite Caliphs (Umayyad Caliphs) affirmed that Isa/Isu (Jesus) Ibn Maryam was Abd Allah (servant of Allah) and not Ibn Allah as implied by the Dyophysites. They coined a phrase “Muhammed(um),” meaning PRAISE BE, to commend his greatness – that is, Muhammedum Isa Ibn Maryam. Note that all the early Umayyad Caliphs were either Nestorian Christians or Monophysites.
Around 9th-12th Century CE, the Abbasid Caliphs usurped the Umayyad hegemony and reformulated “Muhammed(um),” a phrasal commendation to Isa Abd Allah (origin of the word Abdullah), into a proper name (MUHAMMED), which subsequently led to the development of Muhammedanism (now Islam).
And lastly, since Muhammed was invented as a phantom copy of Isa (Jesus) and since Isa’s humanity was in doubt, the Abbasid Caliphs embraced aniconism – that is, a proscription against the creation of Muhammed’s image. Therefore, the reason why there’s no original imagine of Muhammed is the same as why there’s no original image of Jesus. Jesus, Serapis, Zeus and Muhammed are one and the same entity. They’re all mythical figures.
It suffices to know that around 15th Century CE, the Ottoman Turks (who favored Monophysitism) sacked Constantinople and took over the Dyophysite Church of Hagia Sophia. The Dyophysites subsequently relocated to the third Rome (the Vatican), where they built a new Church (St. Peter’s Basilica). And the rest is history!