From the 1931 Winchester catalog --
1931 Model 21 Standard Grade, double trigger.jpg
through at least the 1946, the Model 21 Standard Grade came with the slim forearm. At one of the early Vintage Cups at Pintail Pointe an acquaintance picked up an early 3-digit serial number double trigger, plain extractor, slim forearm, 16-gauge for $900. Guess there has been some inflation since then and an increased interest in these early Model 21s. Time was if anyone wanted a Model 21, they wanted the single selective trigger, ejectors and a beavertail forearm.