300 is a 2 seater shoulder wing aircraft, the first of the extra 300 line. 300S is a midwing single seater. The 300L is a low wing 2 seater. Never heard of an Extra 300X, but the 300XS is the S version with larger tail surfaces and some have added a 330 horse engine, hence where the 330 comes from. 330 is not an official name but rather a nickname that airshow pilots use on their modified Extra's. The L version is also larger than the S model, a 40% 300L is about the same size as a 40% 300S. Hope this helps.

-Kelly