The Model 1912 was introduced in August of the year 1912 in 20-ga only.  Winchester very shortly thereafter added both the 16-ga and 12-ga to the line-up in early 1913.  While there are no surviving factory production records to verify the number made in each gauge, I would estimate that approximately one-third of the first 48,000 made were 12-ga guns.  Your Model 1912 was manufactured in December, 1913.

Bert