Back then, there seemed to be a mania for giving 20s designed for the US market 3" chambers. The Japanese did the same thing. All the Ithaca SKB 20ga sxs had 3" chambers. Shooting magnums through a Model100 with 25" barrels (usually weighing less than 6#) might not be very comfortable even if it didn't harm the gun.

Meanwhile, the Ithaca SKB sxs 12s nearly all had 2 3/4" chambers.