As I tried to explain below, the early tractors had a rubber gasket with 4 brass "nuts" vulcanized into the rubber at the corners. The early gasket was thicker and partially split so it slipped onto the cowel with half the gasket above the cowel and half under it.
The little screw cutouts in the corner of the tach hole in the cowel are much larger on the old cowels.
Those nuts are easily stripped out of the rubber making it very hard to remove the cluster without pulling the cowel and using a vise grip to hold the nut from turning.
The later tractors did away with those embedded nuts and just used a flat rubber gasket and the clips you mention.
I had a good photo of an old style gasket with the embedded brass nuts but cant find it now.
His early tractor does Not have the late style as shown in the photo.