First, I'd found what I thought was the correct date in two places: one was on Find A Grave in a photo of Moses Coburn's gravestone, which you can see here. That seemed to be confirmed by this image of the request for a gravestone in FamilySearch's Maine, Veterans Cemetery Records, 1676-1918 file. Both give the date of death as 4May 1848.
When I downloaded the Will and Probate file, there was a problem. Moses' will was written on 12Mar 1846. That in and of itself was not unusual, however, there is this document in the Probate file:
If Moses' son Lot Coburn presented the will as executor on 20Sept 1847, eight months before the date on the headstone, obviously the date in the headstone request and on the headstone is wrong. Moses might have died on 4May 1847 and whoever filled out the request form gave the wrong date. Or he could have died on some other date in 1847 before 20Sept when Lot Coburn submitted the will to the court.
So I'm left with a mystery date, which for the moment I've changed to just 1847 in my database, and a lesson as to the importance of wills and probates in genealogical research.