The nylon block is indeed there to minimise heat transfer between the inlet manifold and the carburettor/s.
I can see an SU HIF6 carburettor in your photos. What type of vehicle was the RV8 in that you removed them from?
In my P6B, there is a metal liner, similar to what you have, but without the tongue. I can only assume that it must be there to create a swirl effect, a vortex if you will, so as to accelerate the air/fuel mixture as it enters the inlet manifold.
It is only a guess, I could be well off the mark
Ron.