They are called "port stuffers". The intent is to reduce the cross sectional area of the intake ports, in effect making them smaller, theoretically increasing intake charge velocity.
It is said that they increase throttle response at low rpm and "crispen" the engine response.
If you look at virtually all of the future "aftermarket" "Boss heads", virtually all of the intake port sizes have been reduced in size for the same reason.
Variations in those sizes show varying results.
The debate often comes in which combination is "better" and then in the definition of what better is. Opinions vary.
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