This stamp is part of a series from 1976 featuring historic currencies of Morocco. This is the only square coin in the series. I should tell you that the green on the original of this stamp is incredibly vibrant and quite a rare colour for this particular Trusty Stamp Book.
Fortunately for this explanation, someone has one of these coins up for sale in Ebay at the moment! According to their sales patter, it is a silver dirham from the Almohad Dynasty. This was a Moroccan Berber-Muslim dynasty founded in the 12th century that established a Berber state in Tinmel in the Atlas Mountains in roughly 1120. During the course of the 12th century, the Almohad extended their power over all of the Maghreb and took control of Al-Andalus, Moorish Iberia (modern Portugal and southern Spain).