Checking on the Home Page under Research Letters shows the PGCA has records for this gun, so you could order a letter for it. Joining the PGCA reduces the price of the letter. Plain Twist barrels, no engraving and a plain straight grip stock show it to be the lowest grade offered in 1879.
The gun looks to be in nice condition for 146 years old. It appears to me from the photos that the Plain Twist barrels got blued at some point in the gun's life. A proper refinish by one of the experts could really make them "pop" --
https://parkerguns.org/forums/showthread.php?t=45062