[wahern]

There was an A6 record type (https://www.rfc-editor.org/rfc/rfc2874.txt), but it was rejected (https://www.rfc-editor.org/rfc/rfc3364.txt) in favor of the original AAAA record type (https://www.rfc-editor.org/rfc/rfc1886.txt).

Christian Huitema is listed as an author on both AAAA and A6 RFCs, so maybe they know why the AAAA identifier was originally chosen instead of A6. Unfortunately, Google doesn't provide any "AAAA" hits for their blog at https://huitema.wordpress.com/.