In the book "Gamla Svenska Jaktvapen" from Jan Tuveson it says that all hammerguns (except for the model 103) are with 65 mm chambers. The 1937 refers to the hammerless shotguns. Until 1937 they were delivered with 65 mm chamber. Which corresponds with a serial number 165900.
The model 103 could be ordered with 70 mm chambers, special order. The number "70" is stamped between the chambers, above the extractor.

Fritz
Weiss Trading