Pretty certain the original cone number is 025127, it's what i have scribbled in my notes from long ago when i was learning about a pair of M500 i acquired. I'm pretty sure no one else had been inside since they left the factory.

Like many things Altec, the spec for the cone may have changed during production, the vendor may have changed. Who knows?

Some specifics we may never find a solid answer for.

As i have learned from Old Guy, when it comes to Altec products there are few absolutes, and numerous exceptions.

Look to the 604-8G for a classic example of subject to change without notice. Originally introduced with metal annular phase plug, and "light" diaphragm. Ended production with a standard diaphragm, and a radially slit, plastic phase plug with no change of model number, or even a suffix change to indicate a design change. They also went out the door with both new style and old style HF covers and foilcals.