Well, two things come to mind.

I seem to recall my number matched NAPA kit did come with that gasket (not the diaphragm, just the gasket). However, in the few carbs I've rebuilt, that diaphragm has been OK, not to mention that's not a part they really expect you to disassemble (though I have to see how it works).

The other thing that comes to mind is my rebuilt carb came with a vacuum passage plugged in the OVCV so the darn thing didn't even work and it didn't affect anything about how the truck ran.

My Truck: 1987 XtraCab DLX 22R 4WD 5 Speed Manual
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"Speed has never killed anyone. Suddenly becoming stationary, that's what gets you." -Jeremy Clarkson