When the Model 21 was introduced to the trade in 1930, no standard variation included an improved cylinder choke. 12 and 16 gauge guns were available in 26" and 28", cylinder and modified. 20 gauge guns were available in 26" cylinder and modified. Your double trigger gun, if equipped with 28" barrels, was described as a stock number 2108 on the October 15, 1930 stock list. I'm not sure when the improved cylinder choke became standard to replace the cylinder. The cylinder was also standard on prewar Model 24 Winchesters.