Back in the day it was normal practice, especially on better quality machinery, to use heat insulating spacer - gaskets on carburettor flanges. I imagine that that is what the original thick ones are. Hafta say that, as I recall matters, they were normally rather thicker in most applications.
Allegedly there were also potential issues with petrol vapour dissolving into and weakening ordinary paper gaskets. Can't say I've ever seen any evidence for this but I do recall obtaining a New Old Stock aftermarket gasket set for "something" in the middle 1970's that proudly proclaimed petrol proof carburettor flange gaskets. As I recall it they looked just like the ordinary paper ones to me, which is probably why the memory has stuck. I imagine that set was actually mid 1950s - mid 1960s vintage.
On the motorcycle side Amal always supplied thicker flange gaskets for the Monoblock and Concentric MK 1 carburettors and, again, specifically warned against using ordinary paper ones. Even though official bike makers factory full service gasket kits frequently came with ordinary looking paper ones.
Clive
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