Pair of Zane Grey Books: The Mysterious Rider and The Rainbow Trail

Title:  The Mysterious Rider and The Rainbow Trail
Format:  Clothbound Hardcovers
Author:  Zane Grey
Illustrator:  none listed
Year:  1921 and 1915
Publisher:  Grosset & Dunlap Publishers, New York
Length: 336 Pages and 373 Pages
Dimensions: 5.25" wide by 7.5" tall
Condition:  Minor cover and spine wear noted especially around the edges.  The Rainbow Trail has a date stamp of April 1, 1943, throughout the book including on the cover, the inside covers, & random pages throughout the book.  I would love to know the significance of why this was stamped all over this edition?  This edition is loose at the spine - it is tightly bound but the spine is loose.  There are two faint signatures in pencil on the half title & title pages.  No other writing noted.  No torn or missing pages noted.

The Mysterious Rider has the following written in ink on the end-sheet opposite the inside cover "Best liked and most enjoyed book that I read in grammer school.  1937".  Note their spelling of grammer!  No other writing noted in this edition.  Book is tightly bound.  No torn or missing pages noted.