With any engine service that required removing the carb, it would have run just fine with or without the shim prior to drilling the idle spoil ports, and guess they felt it was being left out enough that they should force reinstallation of the shim by adding the spoil sport. Sounds like a brute force fix, but 54/55 years later we are still forced to install that shim if you want it to idle correctly with stock setup.
I will also note:
1st: it has often been incorrectly surmised and stated that this shim was simply needed because of the design of the Carter AFB. The ports were actually added to the Carter Carb as a later adaption to the Carb, during late 1962 model year production, to force the entry of the shim, and not that the shim was added to address original design of the carb.
2nd: it has also been incorrectly stated that the shim was simply a heat shield for the carb, however, it was to prevent long term corrosion from the exhaust gases. Heat is a factor, however, simply heating the base of the carb up is not going to make it corrode. It is the constant direct exposure of the hot exhaust gases over time that will make it corrode.
That metal 64 carb ( be it Carter or Edelbrock ) gasket has a very interesting story behind it
Was sure it was installed as a heat shield but to prevent corrosion of the carb base is so much more serious a problem