Impossible to answer without knowing who made it.
Impossible to answer without a DETAILED description of ALL markings.
No published sn data known.
Depending on EXACTLY what you have, a few hundred to tens of thousands.
Made in Italy. 50-150 USD
50-170 usd
I think what you are seeing is in actuality PN which is a proof mark and stands for polvo negro or black powder. It is most likely a pietta or san Marco made gun.
3000$ or more
AFAIK, Colt never made a single shot pistol
There have been many over the years most were "store brands" during the 70's black powder shooting surge with the exception of Ultra-Hi and Miroku they were sold in the US under there respective names.
read what is on the barrel. it will tell you who made...................
There were thousands of black powder guns made in Spain in any given year. These range from very cheap to well made. You did not give us much information to work on- whether it is a rifle, shotgun, pistol, or cannon.