F55y
y (actually, it is a Y) = the series Type (size/model of truck)

F55Y
F = Flint, MI engine plant
55 = 1955 (first series, if it is a truck engine)

Second Series 1955 trucks would have had a year code of 255.

1955-1st series truck codes
1955 car engine codes

Maybe another member knows what the "trailing "y" ("Y") designates as a series Type?