lol..except the same ammo BEFORE it was belted at the factory was belted at the UNIT level from 20 round cartons.Sorry, no, it's not and just because you repeat yourself, it doesn't make it true. Interchangeable, in a pinch, yes, but not exactly the same.
Belted ammunition is tailored to reduce bore erosion, so it uses a different propellant.
And besides, the specifications for .30-06 never specified muzzle or port pressure, so manufacturers are free to use slower less erosive propellants (and bump up muzzle pressure) for belted. And since ammunition packaged in belts almost never was broken down into M1 clips (as clips were never procurable as a separate item), the risk was minimal.
M2 ball is still M2 ball... there is no such thing as M2 ball "machinegun" ammo. It's M2 ball or it isn't.