E means export. Any of the designations can be followed by an E, with the exception of the MG36.
example: G36E, G36KE, G36CE, etc
The full length is just called "G36".
edit: BTW, the designation has to do with the barrel length. The stock is just something that varies with the models as they are all sold with different version stocks. A KE might have a slightly different stock than the K, which is what distinguishes it. Example: The C <i>might</i> have the same stock as the KE, but is a C due to the barrel length.
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