I was on eBay and found this listing: https://www.ebay.com/itm/224659647675?hash=item344ec22cbb:g:8GkAAOSwVVdhdM6J The M denomination is on the same side as the emperors' portrait. I admit I'm not an expert, but I've never seen that before. Pretty cool.
Those are not the current emperors. That is the reverse. The coin is of Leo IV (775-780) and the obverse has Leo IV and Constantine VI (his son) and the side with the "M" is the reverse with Leo III and Constantine V, grandfather and father of the current emperor. Here is my example: 23 mm. 4.35 grams. Sear 1586. DOC III 4, plate XII.
This illustrates why we need to show both sides of a coin when posting. Certainly I prefer image pairs joined into one but I understand why eBay sellers prefer larger squares to show the better side of their coin so it is up to us to figure out which is which.
Yeah, I have an Irene, (Empress in my avatar), with a reverse of her son and a Large M. It was common in the 8th century.