Your gun has a serial number that falls within the range of the 1849s produced - you can research it based on that (that's where the $300 for a Colt letter comes in).
According to this list, yours was made between 1858 - 1859 (click on the model).
http://proofhouse.com/colt/index.html
I have a Remington-Beals .36 made about the same time that was factory engaved, probably by L. D. Nimschke. Yours is similiar, and definitely worth researching.