The Flues model in 20 ga. was chambered 2-1/2" until the end of production in 1926. The NID was introduced in 1926 and the 20 ga. had 2-3/4" chambers from the start. With the Nitro Special being introduced in 1921 it is possible that the 20 ga. guns may have been 2-1/2" for several years.