How Did This Cross The Atlantic ?

I'm trying to find out how this cover crossed the Atlantic. It looks like it is postmarked DERRY MA 6 18?3. Someone has penciled in the year 1843 on the cover.

It is manuscript rated at 1s/2d in red so I figure it came on a Cunard vessel to Boston? My book on the Cunard sailings doesn't have a date that matches up with the MA 6 March and May, the departure date out of Liverpool is the 4th of the month.

Anyone can provide info on this cover? especially the ship name.

It is a very nice cover for research...
If, as you think, it came from DERRY (Ireland) it would have then gone to Dublin and then on to Liverpool... all of which should have been costed at one shilling ( 1/- ) prepaid for the inland cost plus the ship. The red ink indicates prepayment. I believe the backstamp would be the arrival date at Liverpool (March 7th) and a transAtlantic crossing would be the next one after that date? If Cunard Line then it might be the Caledonia, the Hibernia or Cambria going from Liverpool to Boston via Halifax?

edit...oops, too early for Hibernia as it did not sail until April 1843

Incidentally the postmark is DERBY as the Baxter family has/had family connections there... so the DERRY, DUBLIN, LIVERPOOL route is not valid now.