From the introduction in 1931, --

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through 1936, the Model 31A "Standard" Grade came with a checkered stock and slide handle. In 1937, Remington introduced the Model 31 AP "Standard" Grade with a plain stock and ringed slide handle

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and the checkered "Standard" Grades became the Model 31 AC "Standard" Grade --

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By 1939, the checkered "Standard" Grade was gone and the gun with the plain stock and ringed slide handle became the Model 31 A "Standard" Grade through the end og production.