When the changes occurred I can not help you with.
I admire the doublegun for it's mechanisms as much or more so than the Maker, grade, or condition. What I see as the major difference in these two locks is not so much how the sear is sprung, but the configuration of the sear itself. One being a hanging sear, the latter a straight lever. The leverage at the engagement point on the hammers changed greatly. The upper plate is from a Regular frame, the lower a FW frame. Every part of these two locks is different.