Reverse: The largest symbol on the reverse is the letter M, Greek for 40 (Nummi), being the face value of the coin which equated 1 Follis. The Cross right above and the asterisk to one side are known as Christograms with the latter being an abbreviation of Chi-Rho. This symbol has always been closely associated and is perhaps iconic of Eastern Orthodox Christianity.
At the centre of the letter M, one can see the traces of a now illegible workshop number, and CON below, in what is known as the exergue. This too is a mint mark and defines the origin of the coin as having been made in the Constantinople Mint. The image above shows the coin value in red, the mint marks defining origin in yellow / workshop reference code in lime green (illegible in this case), and the Christograms in orange.
12 Nummi Coin aka 1 Dodecanummium from the reign of Emperor Heraclius (610-641 A.D.) A very small bronze coin measuring approximately 17mm & weighing around 5.45gm, made at the Byzantine mint at Alexandria, Egypt. It is interesting to note that in Egypt the monetary system was based on such Dodecanummium coins, distinct from the 40 Nummi (Follis) of the rest of the Empire. The obverse shows two Emperor busts, depicted facing forward and wearing chlamys and crowns with crosses. The larger figure is that of Heraclius, while the other is Heraclius Constantine. The reverse features a Cross Potent over three steps at the centre, an allegory of Christ on the Golgotha, flanked on each side by two large letters - I B and designate the coins face value: In Greek the letter I represents 10, while B is 2. Hence IB is 10 & 2 (12). Towards the bottom of the coin, in what is technically known as the exergue, one finds four Greek letters - A (alpha) Λ (lambda) ε (epsilon) Ξ (xi) hence ALEX, standing for the mint of origin (Alexandria)