The earlier 20 ga, and 16 ga. did not have any chamber markings. The 20 ga. was 2 1/2" and the 16 ga. was 2 9/16". The 20 ga. was the first to show the new chamber length of 2 3/4" sometime in the 1930's. The 16 ga. was around 1940, although some here have ones a few years earlier with a 2 3/4" chamber marking. My 16 ga. Field circa 1939 still has 2 9/19" chambers, and has the Single Sighting plane which was a $5 option then by 1940 it was standard. If memory serves me I only think they stamped the barrels for chamber length for a few years. When Marling bought them out it was line stamped not in an oval.
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