Many of these small cents exhibit what is known as Longagre doubling - a design element included in most of James Longacre
's engravings. That could be what we're seeing here although MD is a possibility. I hope others can shed some light on this, but in my opinion these "shelves" were engraved into the die. I base this on the fact that they are showing up fairly uniformly across most of the devices.
The date, being hand-punched into the die rather than a part of the obverse design, probably is exhibiting MD. Remember that I am not a FE expert!