they were in the Pacific Coast Conference (PCC). Most of the later Pac-8 was in that conference, plus Idaho and Montana (until 1950). In 1959, the PCC disbanded.

From 1959 to 1968, USC was part of the Athletic Association of Western Universities (AAWU), along with California, UCLA, and Washington. Stanford and Wazzu joined later, and people started calling it the Big Six. In 1964 the Oregon schools joined, and in 1968 the AAWU re-named itself the Pacific 8.

So technically, #3 could be expanded to say that it also excludes the years it was called the AAWU, and the decades USC spent in the AAWU's sort-of-predecessor conference (the PCC). Also, USC has spent much more time in conferences with Montana and Idaho than with utah.