I think that the internet, digital photo's and macro lenses will reveal unusual thimgs about
ancient coins that were rarely seen and perhaps never discussed before.
Here is my wild conjecture: The little bumps were made deliberately when an engraver
had to reduce the amount of relief on the coin. Hence cut back the flat portions which stike the coinblank to make the
field. Easy if you have a computer guided mill but I suspect the depth of metal to be removed was pre-set by making numerous strikes with a drill or more likely an awl. Then he would remove metal with a gouge eventually burnishing away the awl hole. Except in this case he decided that close enough was
good enough and sent the die back
still with shallow perforations in the die
field - which created spikes or bumps in the coin
"When you have eliminated the impossible you are left with the improbable"
Regards, dave