The small pins are on the rear of the cylinder to hold the cylinder in a "safe" position.
The nose of the hammer has a slot in its face and that slot fits over the pins.
This was Colts way of pleasing the Army who insisted that any revolver they bought must have some sort of safety.
When Remington made their Army revolver, they cut slots that the nose of the hammer could fit into in the rear of the cylinder.
If you haven't bought it yet and if the action works like it should, if you can afford it, by all means, buy it.
The lack of markings other than "Samuel Colt, New York" is interesting.
It makes me wonder if someone tried to make a forgery?
A real 3rd model Dragoon would have the following marked on the barrel:
"ADDRESS SAM COLT-NEW YORK CITY"
Colt only made about 10,500 3rd model Dragoons with the serial numbers starting with 10200.
That would make the final guns with serial numbers in the 20,700 range. (Flayderman's Guide says the highest serial number was around 19600).
The newly made 1971-1982 Colt pistols 3rd model Dragoon serial numbers started with 20909 and ran up to 34500.
I don't have the serial numbers for the Colt Signature series that followed the new Colts but they would have been higher than 34500.
In 2003, a Colt Signature 3rd model Dragoon in 95% condition would have been worth $350.