If it is a Joseph Lang the gun would date from about 1874/75. 1870 was gun 3851 and 1875 was gun 5125.

That gun does look crude compared to my Lang #32XX completed in 1866, though that could be from poor treatment and storage. My barrels are Damascus replacements by James Woodward. It only has the "crowned V" view mark and serial number on the action flats - the barrel carries the bulk of the proof marks.