12.5" mid-length gas port size

[Mhan7]

A gas port with 2 different size steps, creating a different gas flow dynamic. Maybe, but it is a new one to me.

Not necessarily a *different* dynamic, just a *desired* dynamic. Based on the dates that you gave for manufacture (you mentioned the 90s), it would have been before adjustable gas blocks and adjustable mass were well known, if even a thing at all. Regardless of whether my guess is even in the ballpark, I'd bet dimes to dollars that it's a thermodynamic consideration of some sort, primarily.

But like I said... I could be (and probably am) wrong. Just taking a stab at it.