All CJ-3A engine blocks should have the following casting number located on the lower right hand side of the engine block:
It's important to note that not all casting numbers are the same. The first number, 641087 is common to all CJ-3A engine blocks. I have seen two versions of the casting numbers with "W11" and "W12." The theory is that this is the forge or cast number. NI is Nickel and CR is Chrome. These two metals are used to give the Iron in the casting alloy properties so it can expand and contract and be machined to function as an engine block. Back in the early days of the automobile many inferior engines were produced because they did not have the proper type or amount alloys in them. Automakers marked their blocks to show what different alloys were being used. This is holdover from those days and even GM into the 1960s marked different alloy codes into different engine blocks for identification of properties and uses. I do not know what the final "N" and digit represent. Nitride has been suggested."
The engine serial number can be found on the waterpump boss at the front of the engine:
From my research, the engine serial number should begin with 3J, and be followed by the serial number, so in my case, 3J39505.