Summary of reasons why this coin must be a portrait of the Prophet Muhammad:
In 693 CE, the Byzantine Empire for the very first time issued coins that had Jesus on the obverse, instead of a portait of the reigning emperor or him and his heirs. This image of Jesus was no mere common religious image: This was a copy of the Divine True Image of Christ taken from the sacred relic, the Cloth of Edessa. The very image on the Byzantine coinage was in and of itself a kind of sacred relic, an "image not made by human hands." This would have caused a serious problem for Muslims: The Byzantine coinage that was in official use by the Caliphate now bore an idolatrous image, a miraculous portrait of Christ as Lord, a kind of holy icon itself worthy of adoration. The legend on the obverse even explicity said "Jesus Christ Lord Savior King of those who rule." Muslims of course were commanded to destroy all idols and idolatrous images
This created an immedate need for the Caliphate to issue a coin suitable for Muslims to handle.This substitute for the True Image of Christ was a portrait of the final Messenger of God, in an clearly human fashion, flanked by his closest companion and brother-in-law, and his wife, with the figures shown almost the same size as him. The legend on the obverse even is a part of the very creed of Islam, the Shahada: "Muhammad the apostle of God", a direct response to the part of the Nicene Creed of Christianity paraphrased on the obverse of the Byzantine coin: "Lord Jesus Christ ... his kingdom will never end.."
The single pillar on the reverse vs. the cross potent on the reverse of the contemporary Byzantine coinage. The single pillar on steps mounted by a ball, is a symbol of the central tenet of Islam, the unity of God. This is opposed to the three arms of the cross with bars mounted on steps, and also ending in balls, in addition to being a cross are also a symbol of the Holy Trinity.
The central figure both grasping a sword and gesturing as if preaching, as opposed the the Sign of Blessing made by Jesus on the Byzantine Coin. This explanatory gesture on the Muslim Coin is appropriate to a Messenger of God, but not to a portrait of a Caliph.
The need for an iconographic coinage acceptable to the Byzantines to pay the required tribute to the Byzantine Empire after the defeat of 689 CE, yet one that was distinctively Islamic.
The obverse inscription which says "Muhammad the Prophet of God" but which makes no reference to the ruling Caliph as on all later Muslim coinage.
The unique nature of this coin, with the image being entirely different from that of the contemporary "Standing Caliph" issues, both in the portait of the central figure, and the presence of three figures on the coin.
The complete withdrawal from circulation of this issue upon pain of death just 2 years later.The severity of this decree would not be fitting a mere portrait of a Caliph, but points to a new, total restriction of religious imagery of any kind as idolatrous.
The accordance of the features of the portrait of the central figure with the descriptions of the Prophet Muhammad from the Hadith. e.g. the full beard. This is modelled on the Byzantine coin portrait of the Emperor Constantine II, yet the figure is grasping a sword in the right hand as opposed to a globus cruciger, a Imperial and religioius symbol of universal rule.
The female figure is veiled. At the earliest stage of Islam, only the Prophet's wives were explicily required to be veiled.
This is also a counter to the Shia trilogy of Muhammad, Ali and Fatima. There is the presence on the coin of the first Caliph Abu Bakr, instead of Ali, who was regarded as the first legitimate Caliph by the Shia. His daughter and Muhammad's wife Aisha was the rallying point of the Umayyads and directly opposed the right of Ali to be Caliph in the struggle between them. Caliph Abd el-Malik's father himself, the Caliph Marwan ibn al-Hakam, was a key ally of Fatima's in the battles of the First Islamic Civil War.
There is a portrait of Abu Bakr and his daughter Aisha vs. Abu Bakr and his other daughter, Aisha's sister and Abdullah ibn az-Zubair's mother Asma. The revolt of the anti-Caliph ibn az-Zubair in Mecca, Abu Bakr's grandson, after twelve years, was finally supressed the year before in 692 CE, This recapture of the holy city of Mecca resulted in the final complete reunification of the Islamic Caliphate under the Umayyads.
Finally, the most compelling reason: A woman bearing a sword - a portrayal of a woman unique in the history of Islam. A fitting portrait for the Prophet Muhammad's wife Aisha, who played a pivotal role in the civil war between the Caliph Ali and the Umayyad governor of Damascus Muawiya, particularly at the Battle of the Camel in 656 CE, a battle which was explicitly [but indirectly] named for Aisha, who personally played a key role in this battle.