Hall 'big bore'.
Only bad points are;
1. That 2" collector size is too small for the 4 into 1 design, apparently worth at least 25HP restriction in comparison to swapping out for a 2.5" collector and subsequent tubing to the muffler.
2. The individual headers tubes are apparently prone to warping. When you remove them they can spring and are difficult to remount (although I've not had any problems with mine). The recommended cure is to have a reinforcing web welded in to join them akin to the more modern alternatives.
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