The third marking is the car line, A - Ford (and FWIW Z - Mustang).
For anyone's future reference:
1st Digit: Decade
A - 1940-'49
B - 1950-'59
C - 1960-'69
D - 1970-'79
E - 1980-'89
2nd Digit: Specific Year of Decad
Last digit for the year, i.e. E2xx would be 1982
3rd Digit : Car Line
A - Ford
D - Falcon
G - Comet, Montego, Cyclone
J - Marine & Industrial
K - Edel
M - Mercury
O - Fairline, Tori
S - Thunderbird
T - Truck
V - Lincoln
W - Couger
Z - Mustang
4th Digit: Engineering Department
A - Chassis
B - Body
E - Engine
Service Part# replace the fourth digitfor the following divisions:
X - Ford Division
Y - Lincoln-Mercury
Z - Original Muscle Parts
M - Ford Motosports SVO.
6015 is the basic part number which is the same for all pieces. So all engine blocks will carry the 6015.
Any set of letters after this indicates a revision of the design process. "A" usually indicates the original design. Going through the alphabet for each revision. The letter "I" is not used however to avoid confusion with either the number "1" or the lower case letter for "L". After "Z" Ford doubles the letters so after "Z" the next revision would be distinguished as "AA" than "AB" and keeps continuing up and beyond even three letters ("AAA", "AAB"...).
There should also be a casting date code, probably by the starter on this engine. It's in the format of number-letter-number-. The first number is the specific year of the decade (decade from the first numeric of the casting number; this is supposed to be a fool-proof feature), the letter is the month starting with A = January, and the last two numbers is the actual day. If this is casted onto the block then this was the day the block was casted. If this code is stamped or inked onto the block than it was the date of manufacture or assembly. If any work has been done to the engine than this number is usually milled off and may or may not be replaced with a new set of alphanumberics.
