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They were used between 73-80 on 350s and have an angled plug, supposedly hard to find and are high-performance heads for 300hp+ engines.
Known as a 'Turbo' head available from GM over the counter.
I found some flow data on slightly ported ones:
.100 lift = 96.39cfm
.200 lift = 143.85 cfm
.300 lift = 190.14 cfm
.400 lift = 218.51 cfm
.500 lift = 243.59 cfm
.600 lift = 261.58 cfm
.700 lift = 271.18 cfm
This is at 25" on a 4" bore.
Not very much by todays standards but realize these were probably done around 1975, and have been hurt and repaired many times over the years, and the valves are pretty sunk, which hurts the flow a lot.