Your extensions look good so far. The .NC and .EC extensions refer to the
preprocessed .GOC files. During compilation of a GEOS program, the
preprocessor GOC takes the .GOC files and turns them into one of two files:
.EC, or .NC. These stand for, as Max said, Error-Checking, or Non-error
Checking. What GOC does is take the GOC code in your .GOC file and turn it
into straight C code. If you're compiling the error-checking version of your
app, you'll get .EC files; .NC if you're compiling the non-error checking
version.
So, to add to your list:
.EC - Error-Checking preprocessed GOC code.
.NC - Non-error Checking preprocessed GOC code.
Nathan