The EBAY seller answered the question
The barrel is marked " model 176 12 ga. Magnum" It is heavier than most barrels I seen, but a 12 ga. round fits perfect.
More info on the 176 barrel. These were made in 10 ga, 12 ga. , and 20 ga. from 1977 until 1985. Barrel lengths were offered in 32- 36 inches, all were heavy barels.
That would be too old to fit a SB-1 frame wouldn't it?
Robert
:-)