Many of the articles I read simply say he put "meat " between the bread. However, this article (http://www.npr.org/blogs/waitwait/2010/11/29/131670129/sandwich-monday-the-sandwich-that-may-or-may-not-have-started-it-all) says it was Salted beef-(Corned beef) which fits in with the era. Salt was used as a preservative.