Content-Transfer-Encoding list registry

Hello all and apologies if this is off topic, but not quite sure where
to ask the question.

In working on my Ruby mail generator / parser
(http://github.com/mikel/mail) I ran into this inside of 2045 part 1:

     encoding := "Content-Transfer-Encoding" ":" mechanism

     mechanism := "7bit" / "8bit" / "binary" /
                  "quoted-printable" / "base64" /
                  ietf-token / x-token

     ietf-token := <An extension token defined by a
                    standards-track RFC and registered
                    with IANA.>

The only IANA public list I can find is:
http://www.iana.org/assignments/transfer-encodings/ which matches the
list already defined in mechanism.

Is this the only list of this type?  Or am I missing something?

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