When were briefs popular outside of competition or lap-swimmers?
Looking at this archive of Jantzen ads in this web site, it seems that 1970's or 1980's had most ads for briefs for mainstream (probably stuff you buy in department stores), plus there's an ad describing Lycra
http://www.advertisingarchives.co.uk/index.php?service=search&action=do_quick_search&la nguage=en&q=jantzen