I am doing a quiz on fictional detectives and the clue is 7th son of 7th son foils scheme to kill trial witness on punchbowl hill, and, I therefore wondered if Charlie Chan could be the answer, with the Punchbowl Hill connection.
The only detective of note that seems to fit the clue given would have to be Charlie Chan, although I do not recall him ever being refered, either by himself or anyone else as "the seventh son of a seventh son." If such a reference is to be found, and it would have probably been in a passing remark, it would have most probably have been made on one of the stories written by Charlie Chan's creator, Earl Derr Biggers.
The Punchbowl Hill reference also points to Chan, although his connection with this Honolulu location relates to his home, located on the gentle slope of the extinct volcanic crater.
I feel that the clue provided to you was not meant to be taken as exact information, but, rather, as a tool to point you in the right direction...Charlie Chan.
Maybe it was Charlie Jr. that was being reffered to , although It seems to me he was probably # 3 son , even though the Monograms obviously refer to Tommy as the # 3 son ... but going back to the Fox's ....