Bulletin 08-5a-76 Sept 20 1976 Metal gasket insert. What this did was raise the top of carb about 2/32[guess] above its original position above bowl. The holley 1945 was an air bleed carb. This raised the air supply tube to the main well higher above the fuel. Which got those cross flow air holes above the fuel level, so they could let in more air and less fuel thus leaner air fuel ratio. That is the simple story

That picture is early carb 1975 First hole at 23/32, only 3 holes on 4 sides ,total of 12

That green dot may have been original fuel line,just a guess .

First hole 25/32 lots of holes,bigger and go to bottom ,later model 1945