California elementry school testbooks frrom the 1920s

This is a childhood memory of my mother's, so I don't have much to go
on. It was a set of books, rather small in size and white in color.
There was one book for each country (in the world? in Europe?) I'm
guessing that they were published around 1920. She recalls that the
book on Germany talked about Big Bertha, and the book on Belgium
described big shaggy dogs that pulled milk carts. 

That's all I have. If anyone knows what I'm talking about, I would
really appreciate any information on title, author or publisher If
this is the wrong place to ask, I apologize for wasting your time.

TIA
Joan