This is a silver tetradrachm of Alexander the Great (336-323 BC) which was minted during his lifetime and rule , minted between 325 and 323 BC in either the city of Myriandrus (in Cilicia) or possibly Issos.

Head of Herakles right,   wearing the Nemean lion skin headdress. Reverse: Zeus seated  legs together holding long sceptre in left hand and and eagle with closed wings on the right hand .Inscription ALEXANDROU meaning 'of Alexander'. To left, monogram above club in wreath , below throne mononogram of MI

Most, though not all, lifetime Alexander III tetradrachms like this one, portray on the reverse Zeus with open legs and a reverse inscription that reads "Of Alexander," not "Of King Alexander" (i.e. no royal title).

The other features of this coin which strongly suggest it is a lifetime issue is the relatively small flan (and consequent low weight)  Furthermore I bought this coin from the well known coin dealers B A Seaby in London in 1968 on the understanding that it was considered to be a lifetime issue. I still have the coin ticket!

Lifetime issues of Alexander the Great's silver tetradrachm, are those struck during his reign from approximately 336–323 BC are estimated to comprise less than 10% of the total known examples in museums, private collections, and on the market. This figure is derived from die studies and hoard analyses in key numismatic catalogues, especially  Martin Jessop Price's Coinage in the Name of Alexander the Great and Philip Arrhidaeus (British Museum, 1991), which documents over 2,500 distinct types across more than 120 mints. While the exact proportion varies by mint (e.g., higher at early Macedonian centers like Amphipolis, lower at later eastern ones), the consensus across scholarly sources emphasizes the relative scarcity of lifetime strikes compared to posthumous issues, which continued production for nearly two centuries under the Diadochi and later Hellenistic rulers

 The <10% estimate also holds across sources like the American Numismatic Society's PELLA database and Reddit's r/AncientCoins discussions among specialists.

Price: 675 GBP