And most important, few but Professor Ettinger had seen the letters.
I am not sure what the phrase "few but" exactly means. I tried to find the meaning on the internet but without any success. Given the context (Ettinger actually read the letters) I would understand it that Ettinger belonged to a minority of people who head read the letters. But if I apply to this phrase the same logic as to the similar one "all but", the meaning is rather different: few people read it but Ettinger not. So could you tell me which meaning is correct.