Current Crochet Thread sizes vs. 1940's sizes

My mother-in-law is going to make a crocheted tablecloth for us from a pattern that was published in 1946. It calls for 68 balls of Aunt Lydia's size 30 crochet thread. We're wondering if size 30 in 1946 is the same as the size 30 that is available now. We realize today's sizes are standardized, but were the same standards in effect in the 1940's? We've tried to find a way to contact Aunt Lydia's, because this thread is still sold in stores, but we can't find an email address or website for them (just sites where we can buy it). Also, we don't know how much thread was on a ball in 1946. Were they 500+ yards like they are today? Does anyone have any experience relating a 1940's pattern to the thread that is available in stores and online today?