Tooth brushing was common by the 1930s, after which toothpaste sales leveled off even though the advertising, of course, continued. 

【翻译】到了20世纪30年代，刷牙已经司空见惯，从那之后，牙膏的销售就趋于平稳，尽管广告还在继续。

after which...已经明确告诉你：这是定语从句。怎么会是名词性从句呢？which 是干什么的？在没有疑问概念的句子里面，它能引导名词性从句吗？

which 是关系代词，其先行词是the 1930s。因此，after which = after the 1930s。

定语从句成分分析：

toothpaste sales （主语）leveled off （谓语）even though the advertising, of course, continued. （让步从句）