Early Double-Action and the KISS Koncept

Never, ever lose sight of the brilliant reality put forth by the great Kalashnikov in the quote above.

Keep It Simple, Stupid. Just in case some haven't heard of the Koncept.
We're going to get into the way-back machine now; back to that magic time when things moved at a walking speed. That is to say: The black-powder era, specifically the percussion cap era.
Above, the action of an under-hammer pistol.
First thing one notices is that the trigger guard and the mainspring are one and the same unit.
Sweet, no?
Better than that; there are only two (Yes, two, count 'em, two) moving parts: the hammer and the trigger.
It ain't rocket science. The hammer is pulled back and the trigger engages the half-cock notch and then the whatever-the-hell-that-notch-is-called... notch.
Obviously though, this ain't double-action but we'll get there.
The under-hammers seem to be an under appreciated form of single shot pistol although they weren't all single shot as shown by the Blunt and Syms pepperbox pictured below.
When I first saw one of these, my first thought was this: "Your cap will fall off!". Your percussion cap - not the one on your head. well, maybe not. Even more bizarre, the first underhammers were German flintlocks.
Okay, upon firing, the frizzen opens and the sparks have to get up to the priming charge before it falls out.
Apparently, it works - even if in a non-zero-gravity environment.
The only disadvantages to the underhammer system, to my mind, is the cap retention and the fact that you'd have hot little bits of cap hitting your hand.
On the other hand, they're not hitting you in the face - not to mention, the top of the barrel is clean from breach to muzzle with no chunks of mechanism protruding onto the sight picture although sighting probably wasn't an issue with the pocket pistols. Also, it's less likely to hang up on ones pocket in being drawn.
There are anoraks who, like those who follow every other oddball thing in the world, have a website discussing their particular, sick obsession. The Underhammer Society.

Some simple rambling through a few different designs:
The Correvon pistol of one century before my blessed nativity.

But, saving the best for last; below;
The Copper underhammer, pocket pistol.
According to the anorak site linked to above, this one isn't marked as a Cooper but the action, such as it is, is identical.

I shouldn't even have to explain this. Pull the ring-trigger back and it lifts the hammer/main spring until... it doesn't. Then, BOOM.
So simple and elegant it makes you wonder why anyone ever bothered with any other design.
No safety notch but I can't see why it would need one.
Also, notice that the pocket pouch holds all the dangerous bits in place until it's withdrawn.
Okay, I don't want to piss in the Cheerios of all those AR15 shooters and power-window aficionados out there, so I'll neutralize this with the following.

Shit-Oh-Dear.
A 24 shot, percussion revolver.
Here's where the nightmare of the falling caps would seriously manifest itself - not to mention the simple (tedious) process of sticking all those damned tiny things onto their nipples.
Notice though, the loading process is streamlined by the loading lever being forked so it tamps two cylinders at once.
What a time-saver.
You'd still be all day loading the damned thing.
No wonder it carried 24 shots.