The model 5100 Stevens shotgun has a plastic stock
Try e-gunparts.com
model 94B Stevens plastic stock was mfg in 1950. Stevens uded letter code starting with A in 1949 until P in 1969.
i have had several and was told they were likely made during WWII when walnut was being used for military purposes
gun-parts.com has them...but for 12 ga only
1940's
Try e-gunparts.com
Any of the 124 series.
Try e-gunparts.com, Shotgun News, Gun List, do a web search on "shotgun stocks".
1950
http://www.gun-parts.com/index.html/
Remove buttplate. Use a socket extension and rachet to unscrew the stock bolt (it's in that hole in the center of the butt)
$100-$175, depending on condition.