I checked in Schwing and could not find evidence that there was a separate serial number range for the three barrel sets, but I will keep looking. I may have been absolutely wrong about that. Your gun is in the 20 gauge serial number range for 1959, the first year that the 28 and .410 barrels were made. In 1960, the first full NSSA skeet year that Bob Rodale would have shot this gun, he had a .945 average in 20 gauge, .943 in 28 gauge, .878 in the .410. Those averages don't sound too impressive in today's world, but in 1959, they were quite respectable.