Pliladelphia Mint Marks On Pennies

2017 was the only Cent that had a P mint mark.
They did it to commemorate the Philadelphia Mint's 225th anniversary.
Aside from War Nickels, no US coins bore the P mint mark until it was added to the 1979 SBA Dollar, and all other denominations the following year, EXCEPT the Cent.
Before branch mints were opened, no US coinage had mint marks. When some opened, they had mint marks added so they could be accountable. But from that point, the lack of a mint mark was understood to mean the coin came from the 'mother' mint in Philadelphia.

Looks like the first branch mint went online in 1837, at Charlotte, which only produced gold coins.

You missed all denominations lacking mint marks from 1965 to 1967, jbuck.
But regardless, most people didn't in these exceptions that plain coins were minted elsewhere, and, as had long been the tradition, thought they were Philadelphia products.