340 block- Casting #s, Vin#s, And Assy #s
This number on the block, under the driver's head, from what I understand...........
The letter in front of the "W" denotes the build date.
D = 1968
E = 1969
F = 1970
G =1971
Etc.
Maybe the M is for Marine, or replacement? Just spit balling.