This is a quick block ID for junk yard hunting, etc. You must find the date code at the back LH side of the block first. The letter is the month, next 2 digits are the day and the last digit is the year. Here are 2examples:

A249 code "XE"
A = January - 24 is the 24th day - and 9 is 1969
The block is a 69 428.

Another example is:

K109 "WA"
K = October or November - 10th day - 9 = 1969
The block is a 70 455 4 bolt.
These motors usually have a large "70" at the rear of the block.

*Note - blocks with only 2 mounting holes or pads (for the motor mounts) per side is from 1969 and older blocks with 5 or 3 mounting holes or pads (for the motor mounts) per side is from 1970 and newer blocks with 2 freeze plugs per side is 1966 and older blocks with 3 freeze plugs per side is 1967 and newer certain blocks for 4 bbl carb usage have scallops in the cylinders at the deck for unshrouding the intake valves. Most 1967 and earlier blocks place these scallops at approx 4 o'clock and 8 o'clockwhile 1968 and newer blocks place the scallops at 3 o'clock and 6 o'clock . Early 2 bbl blocks and most newer blocks did not have any scallops.

Service Replacement (SR) blocks do not always have a code stamped on the front of the engine. They do, however, usually have a casting number. This casting number is usually on the RH side of the rear portion of the block - where the transmission bolts up. 
Here they are: