Anybody familiar with old percussion pistols,derringers? Picked this up in a lot at an estate sale. Not sure if its really something or a retro?. The trigger is functional and there is a number "6" stamped underneath in front of the trigger guard and on the barrel. Overall length is 6.75" and barrel is 2.75".Thanks

Looks like either what was called a "muff" pistol or a "boot" pistol. Common in England and Scotland.

Sounds and looks like a repro.

I'd guess a modern repro or a late Belgian or Spanish made edition.
Commonly listed as a Boot Pistol.

CVA, Pedersoli and others sold kits and complete guns over the years.

Many of these are 'Screw Barrel' actions.
The bbl actually unscrews from the short frame at the joint you can see just in front of the hammer.

Octagon shaped bbl pistols were provided with a small box style wrench
to slip over the muzzle to tighten and loosen the bbl.
Round bbl versions used a square tip 'Allen' style wrench that fit inside the muzzle a short distance.

Once the bbl is unscrewed,,you simply drop a round lead ball down into the breach end which is chambered a short distance so the ball drops in and stops against a shoulder.
Then fill the chamber behind it full of BP. The carefully screw the frame back onto the bbl.
The pistol is loaded.
Cock the hammer and cap it. You are ready to fire.
No ram-rod needed

The gun looks heavily polished and probably hot salt blued. Lots of tool marks, no markings or more importantly,,no foreign proofs. That makes me think it may be one put together as a kit.
Of course it could have been an earlier foreign mfg piece that someone refinished and scrubbed any markings off of in the process.

Most of these are smooth bore,,short range point & shoot.
But they may be rifled as well.

The same screw bbl action idea has been used on some very light weight small cal rifles and fowlers as well.