As of this time three small leatherette titles have been seen in this format.
A Shropshire Lad
The Rubaiyat of Omar Khayyam
As a Man Thinketh

The books are 5.75″ x 3.75″. There is a small picture tipped in opposite of the title page. No ads are noted in the book.

No known Altemus series contains these three titles. I suspect these volumes came boxed and that the box will identify the series name.