What susjwp said. Check out the "Design and Engineering" section of the FAQs
https://lcsca.clubexpress.com/content.aspx?page_id=274&club_id=43784

The very short version is that the FW frame was designed for the introduction of the 20g Smith in 1907. The shipping records do not establish when the first 16g and 12g guns were made with the FW frame; but definitely pre-1913. And BTW some R frame 12s and 16s are slightly lighter weight than FW.

The very short version of Smith perceived desirability/quality is the pre-1913 No. 00 probably had better fit and finish than post-1913 Field, which had better fit and finish than 1940s and Marlin era guns. Things were booming in the 20s after the Simmonds family, owners of Simonds Saw and Steel Co., acquired Hunter Arms and Stephen Gilles was named General Manager of the operation, and the guns produced then were very well made.
Marlin did thicken the head of the stock wood and bevel the lockplate to deal with the 'long cracked Smith' issue.

https://sites.google.com/a/damascusknowledge.com/www/home