Pahlavi was the liturgical and official language of the Church of Fars for some time. The Chronicles of Seert narrates that in about AD 470, Bishop Mana of Rewardushir wrote religious discourses, canticles and hymns in Pahlavi and translated the theological works of Diodore of Tarsus and Theodore of Mopsuestia from Greek and sent the copies to the islands of the sea and India.( A Mingana, Early Spread of Christianity in India, The bulletin of John Rylands Library,Vol 10 p 460) The findings at Turfan confirms these citations in Chronicles of Seert.
This simply shows the hierarchical relation of the Church of India to the Church of Fars. This hierarchical relation continued until the Indian Church was also elevated to a Metropolitanate by Patriarch Isho- Yahb II (AD 628-643) and came directly under the Catholicose Patriarch- the Metropolitan of India cannot be under another Metropolitan, but to the Patriarch directly. ( A Mingana, Early spread of Christianity in India, Journal of John Rylands University Library, vol 10 p 496-7)
The interpretations of the Pahlavi inscriptions on these crosses have been studied by a number of scholars as I have described in the article, from Burnell in AD 1874 to Gerd Gropp in AD 1970. CPT Winkworth has done extensive comparative study on these inscriptions and reported for the first time in AD 1929 that these are copies and he opined that the Mount cross could be the model for the rest of the copies depending on the epigraphic evidences.
Later, B T Anklesaria, an eminent Pahlavi and Zoroastrian scholar compared all these cross inscriptions including the Alengadu cross which was discovered only in 1931 and published a paper in the Journal of K R Cama Oriental Institute in 1958. He agreed with Winkworth that these are copies and found that the Alengadu cross in the oldest based on epigraphical and orthographical evidences, as one of the words was inscribed perfectly only on Alengadu cross. Note that when Winkworth studied about the inscriptions, Alengadu cross was not discovered yet.
Anklesaria also mentions that the Alengadu Cross is taken from Cranganore which is a revolutionary information to me, as I have been looking for the cross at Cranganore as described by Gouvea in AD 1606 in Jornada. Fr Jacob Kollamparabil reported that the Kottayam Crosses were taken from Cranganore, but it was in AD 1524 when Samoothiry, the King of Calicut conquered Cranganore and burned down all the three churches in there. So, I was sure, the cross described by Gouvea in AD 1606 is not the Kottayam Cross. Unfortunately, Anklesaria does not mention the source of this information.
Obviously, Anklesaria has had the advantage of having opportunity to study all the six crosses of Kerala and the Mount Cross. It seems that these crosses were originally erected on the Malabar coast and then copied to Coromandel coast with local adaptations- the Pallava architectural features of arch, makara torana etc. Among the Crosses in Kerala, the Kadamattom and the larger cross at Kottayam are the only crosses that show similarity to Mount Cross, all the rest are similar to Alengadu Cross.