Early A, 273, 340 manfolds dual exhaust.

Dartswinger70 said

You are probably correct, the advantage i see is, they could swap in a hotter engine or heads later,say a 340-360. Exhaust will already be there.

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And small ports dumping into large ones is better than the other way around. LOL No difference than putting a set of headers on your 273 or 318 to gain some performance.