This number was most likely from the time when the 714 area code covered the area outside of LA, including all of Southern California San Diego county and east, Riverside County, San Bernardino County, and the east side of the Sierras all the way up to Inyo.
I remember that no prefix in Orange County was numbered below 520. All prefixes numbered lower than 520 were outside of Orange county.
Most of the prefixes in City of Orange were, as of the 1970s:
532, 538, 633, 634, 637, 639, 771, 997, 998.
