Original: The Plutonium PAK was also released as an upgrade package to convert the original release of Duke Nukem 3D (v1.3d) to the new Atomic Edition (v1.4, later patched to v1.5).
Simplified: The Plutonium PAK was also sold as an upgrade package to convert the first available version of Duke Nukem 3D (v1.3d) to the new Atomic Edition (v1.4, later upgraded again to v1.5).