The Pettingill Patent Percussion Revolver.
It does actually have an internal hammer - so is fired D.A. only - I haven't found any reviews that assess how it worked or what weight was the trigger pull - but it seems to have proved to be mechanically complex and somewhat unreliable in the field.
ArmsCollectors.com have an excellent detailed photo-story on this novel double-action percussion piece that was produced by Rogers,Spencer & Company of Willow Vale.
First patented by C. S. Pettingill of Connecticut in 1856 (improved 1858 &1862) - exactly 2,001 guns were delivered to the US Government - but it seems that as many as 3,000 total may have been made - judging from the serial numbers.
The Army model was a .44" calibre and had a 7.5" octagonal barrel with six rifling grooves.
There was a shorter 'Navy' model in .34" calibre with a 4.5" barrel.
Unusual Calibre that .34" eh
These guns saw service in the American Civil War - however Rogers & Spencer used the front-end of this gun in their shortly to be developed .44" calibre single action revolver - as clearly shown in the next picture: But the S/A had a five groove barrel.
Rogers & Spencer Percussion Revolver:
Said by 'those who know' to be one of the very best percussion revolvers made - these guns were too late to be used in the Civil War - despite the War Department contract for 5,000 of them. These guns were held in store in New York until they were eventually released in 1901 and auctioned-off in unused condition for around 50 cents each.
- Remember that 1901 starter price next time you try to buy one or even a replica eh.