Bob,

1370 is the weight of the barrels in Kg (a little more than 3 lbs). Each barrel will have two markings on them to determine choke. The link to the gun above shows the an 18.3 and a 17.3 on the left barrel; these are in millimeters. When you subtract them, you get 1mm or .039 inches for the choke. I've seen other early 50s SO guns that were marked with a year vice the roman numerals.

Ken