On a hunch, I decided to check Copper Coins. Many mint mark punches are shared across denominations, particularly in Denver. This mint mark has some similarities to 1979D-1MM-003, which includes the comment (emphasis mine):
So apparently there was at least one D punch in use that was known to have damage to the lower left serif that falsely gives the appearance of a split serif.
Quote:
A very nice rotation can be seen to include splits on both serifs. This RPM is punch on a common damaged punch giving the appearance of D/D/D on the lower left serif.
A very nice rotation can be seen to include splits on both serifs. This RPM is punch on a common damaged punch giving the appearance of D/D/D on the lower left serif.
So apparently there was at least one D punch in use that was known to have damage to the lower left serif that falsely gives the appearance of a split serif.





























