There is this person on a passenger list, no further information. What was your grandfather's occupation and was he married at the time? This one was single.
WILLIAM ARMSTRONG
Age 35
Birth 1875
Single
Labourer
9th April 1910
Departure port Londonderry
Destination port Halifax, Canada
Ship name NUMIDIAN
What was the date on the letter? Obviously if it was after 9th April we can rule this one out.