I have a rebuild kit and there is a gasket in it that looks like it goes between the accelerator pump diaphragm and the bottom of the well.
I have never taken a YF apart which had that gasket in it.
The instruction sheet that came with the kit had a footnote for that gasket. It stated that the gasket could be used to correct leaks in the accelerator pump. It is not a hard gasket and would likely compress enough to seal a corroded well or a slightly warped bonnet that goes on top of diaphragm.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission