Back in the late 1970s, I was able to handle( but not fire!) a DB Tower made shotgun. I examined the locks in detail, and was amazed at the smoothness, and light tension weight of the combined springs and action. Its perhaps the softest feather spring I have ever felt on any gun. Pulling the cock back was smooth, and light- no bruised thumbs like I experienced with so many replica Brown Bess locks, that immitated the same size locks used on that Shotgun.
The Shotgun I handled was made in 1776, and so marked, ( Which is why it was in the gun collection), 12 gauge, with 36 inch barrels. It balanced perfectly where my forward hand naturally held the forestock, and the gun mounted easily to the pocket of my shoulder.
I cannot tell you how fast the lock was, because the timing equipment was JUST NOT available, then. A patina covered the face of the frizzen on top of the scrape marks made when it was last fired. All I could do was feel the difference in the smoothness of rotation of the cock and tumbler, the light trigger pulls, and check to see how much tension kept the frizzen closed. Several experienced Flintlock shooters at my gun club were also able to handle the gun, including 2 men who had built Flintguns. All said that it was the smoothest set of flintlock actions they had ever felt. They said they didn't even know how it was possible to get an action to be that light and fast!The one spring available to them to examine was the feather spring, and both of the " Makers" did so. Both told me that this was far better than any lock currently being made. The upper arme of the spring was shaped so that the spring did not have to be further compressed by the cam on the bottom of the frizzen before the frizzens opened. Since both locks were made the same way, I have to believe this was done by design, and not by accident.
This shotgun had a sling swivel mounted to the front of the trigger guard, and a sling swivel attacked to the center under rib several inches in front of the forestock. I don't recall now if it was attached to the Ramrod thimble or not. Its been a few years, and I just wasn't focusing on that detail.
I had no doubt then that the gun came from the Tower Armory, and was a Military Officer's shotgun. What I found fascinating was that the both the right and left hand large locks were mirror copies of each other, indicating that the armories were quite capable of making a large LH lock if ordered. I know of NO LH Brown Bess guns in existence, and know of no written records indicating they were ever made. But, here in my hands was a DB shotgun with Rt and LH locks that would fit any BB Musket of the day.