From the introduction of the Model 31 --
Model 31 3-shot Brochur.jpg
Model 31 ad June 1931 Field & Stream.jpg
through 1936, the Model 31A "Standard" Grade had a checkered stock and slide handle.
March 6, 1936 Retail Price List.jpg
For 1937, Remington introduced the Model 37AP "Standard" Grade with a plain stock and ringed slide handle.
Model 31AP Standard Grade.jpeg
The guns with checkered stocks and slide handles became the Model 31AC "Standard" Grade --
Model 31AC Standard Grade 8-37.jpg
That continued through 1938. By 1939, I guess they had used up the inventory of checkered guns and the gun with the plain stock and ringed slide handle became the Model 31A "Standard" Grade.
From the introduction of the Remington Autoloading Shotgun in 1905, the A "Standard" Grades had American walnut stocks and the B "Special" Grades had imported walnut stocks. This announcement appeared in the February 15, 1939, Remington Arms Co., Inc. catalog --
Selected Figured Walnut, February 15, 1939.jpg
In the January 3, 1938, price list the Model 31B "Special" Grade had a retail price of $77.45. In the February 15, 1939, price list the Model 31B "Special" Grade had a retail price of $51.70.