In 1967 and 1968, the base engine 327, 300 Hp in the Corvettes had 10.0:1 compression (and only in those two years). I think that these are 461 heads, as were used on the prior 300 hp Corvette engines.
The 461 heads were casted from 1964 to 1966.
The 461X heads were casted from 1960 to 1963.
Over the years one thing I always noticed about the 461's where they had smaller chambers than the later camel back heads.
I don't think they ever came on a engine with less than 10.5:1 compression.
During those years only 2 small blocks were in production, the 283 and the 327.