That SN and London Steel were used on pre-1913 No. 0 and post-1913 Skeet Special and Ideal grades.

Check 'Shotguns' on the Home Page under 'Grades' for illustrations of all 3
https://www.lcsmith.org/content.aspx?page_id=22&club_id=43784&module_id=246415

Here is where the SNs are marked on post-1913 guns.
A 'R' prefix indicates a Regular frame