Are you counting the Maxim variants as the same gun? They use different rounds, so it's technically a different monster altogether.
The PM M1910 is basically the Russian variant of the Maxim. It uses the 7.62x39 instead of the .303 British.
The German MG 08 is also a heavy machine gun variant of the Maxim, but chambered for the 7.92 Mauser instead. There's also a variant called the MG 18 TuF but that one uses a round even larger than the .50 BMG and is used in anti-tank and anti-air roles.
The base Maxim gun is just a general purpose machine gun, even if they didn't have a designation for that when it first rolled out.
Edit: Then again, the Maxim gun was only considered "heavy" due to its cumbersome bulk and weight, not its capabilities.