1539, 1514, etc are just manufacturer date code. Mega328P is the chip, AU is the package type (32 pin TQFP). PU is the plastic DIP, MU is the small leadless package.
1539, 1514, etc are just manufacturer date code. Mega328P is the chip, AU is the package type (32 pin TQFP). PU is the plastic DIP, MU is the small leadless package.