Thanks so much for the info on DLK's death certificate.
As I stated in my post that you responded to, I had already convinced myself that the 1850 date is off by about six years. If you look at all the many censuses he is in, you will see great consistency in his stated ages which support an 1856 birthyear. You didn't state who was the informant for his death certificate information, but it might well be the same person who had his gravestone made. In any case, I'm sure a misunderstanding within his surviving family caused the death certificate and gravestone to agree on the same incorrect birthyear.
If you are a descendant of DLK, I would love to correspond with you and exchange information about others in his family; siblings and ancestors. David was a brother to my ancestor, O N Koon. Would love to hear from you.
Have a great photo of David (and of his father) if you wish a copy.