Mongol - Ghengis Khan dirhem

Purchased at auction from Frank Robinson (114 lot 362; $366), it was described as: MONGOLS, Genghis Khan, 1206-27 [AH 603 - 624], Billon Dirhem, AF/F-VF, one side somewhat off-center, minor weakness, silver color with some coppery toning. New information on the early series of Mongol coins during Chingis Khan [alt. spelling] is still coming out as new finds and scholarship allow. Like most coins of Genghis Khan this example does not bear his Tamgha, a personal symbol found on most later Mongol coins.
This appears to be in the design known as a Ghazna type which refers to the Khan as "The Great of the Great Khagan." Thus it is one step lower than the very rare issues that quote his name but much scarcer than the vast majority that are anonymous. This is a dirhem due to the silver content, other coins with the same legend, that are copper, are generally known as jitals. I admit that these denomination terms seem to be interchangeable depending on who's writing about them.
OB: Al Adil / Al Khagan / Al A'zam [The Great of the Great Khagan]; Rx; Name of the Abbaside Caliph in Bagdad. Album # 1969; Nyamaa # 5.
The following is from Steve Album's "Checklist" [2nd ed.]:
The Great Mongols, Chingiz Khan and his descendants, were not Muslims, but struck Islamic style coinage in the Muslim lands they conquered. They had no indigenous coinage, but some Mongol rulers had earlier produced Chinese style cash in parts of northern China they had occupied [Liao Dynasty]. As a general rule, they adopted the local currencies in each conquered area, changing only the inscriptions to suit the new political order.
Most early Mongol coinage is anonymous, except for mention of the caliph. Each mint or group of adjacent mints maintained its own types and standards. Only the gold coinage is normally dated and often bears the mint name. The silver and copper coinage is usually undated, and some types are only conjecturally assigned to the Mongols...
further... Silver full dirhams were struck in both eastern Khurasan (especially Ghazna, Balkh & Herat), and in the Transcaucasian regions of the northwest (especially Tiflis and Tabriz), alongside half dirhams from the later region. ... Base metal jitals were struck only in eastern Khurasan and Sind, and often contain considerable amounts of lead and zinc in addition to copper.