This must be what Dagwood Bumstead (hero of “Blondie“) uses on his enormous eponymous sandwiches!
Bumstead’s Salt ad from a 1935 Ward, Lock & Co. guide to London, England.
Here is a link to some Blondie comic strips showing Dagwood Bumstead making one of his sandwiches. I was going to put the image in the post but the copyright/fair use stuff is a little complex for me. Perhaps I just need a little Bokar coffee!
Always loved Blondie! Those sandwiches were amazing. How did he stay so thin?