Heraclius and Heraclius Constantine. AE Follis. 610-641 AD. Constantinople. AD 610-641. 32mm. Overstruck on an earlier follis.

No legend, Heraclius, crowned and in military attire, with moustache and long beard, holding long cross, on left and Heraclius Constantine, crowned and wearing chlamys, with short beard, holding cross on globe, on right, both standing facing. Sometimes a monogram to left or right, or a K to right, sometimes a monogram between their heads.
Large M, ANNO to left, cross or cross over C or Θ or a monogram above, regnal year to right, officina letter below; mintmark CON (common) or CONΘ (rare). 

Sear 810. (1,032 combinations of letters, years etc!)

Courtesy of Stoa Image Gallery, album 131, Nov. 2010

This class of follis (9-11g.) was introduced in year twenty (629-
630) in an effort to return the AE coinage to its earlier weight and value. In year 21, the weight fell to 6-8g, two years later to approx. 4.5-6g.

These are frequently overstruck on earlier coins, often of Heraclius' own reign.