Hi. This is Iio.
All of them are specified with RIAJ standard.

<a8dec30.0310070102.759f0a25@posting.google.com>の記事において
nijisei@yahoo.comさんは書きました。

>> Specifically, does anyone know what the (L) and (X) codes mean?

(L) means it was recorded in Japan, and
it has been printed with release date.

If it was recorded oversea, it will be (Y),
printed with released date in Japan and
first released date on the world ( in parentheses ).

(X) means this is not for rent, sell only.

>> Also, I see that there are almost always 2 dates listed.
...
>> I've also heard that it represents the date at which
>> retailers are allowed to sell the CD at a price lower than that
>> specified.

Correct. Known as 'no-discount resale system'.
-- 
sen_nin@st.rim.or.jp