76/77ish M4Ms, what's the triangular flap on front of airhorn?

[Shark Racer]

Just where the front airhorn screw would be - what does this cover do?

I'm sure I've seen the answer somewhere, just having trouble placing it...

[Ethan1]

 I believe that is where the air bleed can be adjusted. Not 100% sure.

[Shark Racer]

Well, I pulled the flap off and it's just a gaping hole that leads into the float area, near as I can tell. Was it meant for some kind of vent maybe? Perhaps it's where the air bleed used to be but is unused on these models?

There was a triangular rubber gasket underneath the metal flap, and it seemed pretty well sealed.

[Ethan1]

 Not sure either, Steve. Maybe someone else will know.

[Shark Racer]

Looking at a vendor's website who sells a replacement plate, they refer to it as a vent. Soooo - there we go. Just a vent.

Wonder what the history of those was. I'd guess they disappeared as part of evap concerns.