Mackaydon said:BTW, the "M" is the Roman letter that stood for the value of the coin; 40 'nummi'. (If it were a "K", then 20 nummi; or an "I" for ten nummi.) The mint location is designated by the letters in the exergue--below the "M"--which I can't make out.
EDIT: I believe the mint mark is THEUP; i. e. Antioch.
THEUP was the abbreviation for THEOUPOLIS (Antioch)