The 20" bass configuration appears in the 1971 catalog, model 987P.
Here: http://www.vintagedrumguide.com/images/ludwig_sets/super_classic/ludwig_super_classic_71.jpg
No 1960s catalog that I've seen has the 20" bass, although there is a time lag between catalogs getting put together, printed, and sent out. The catalog dated 1971 might formalize kits which were being shipped in the late 1960s.
If you search for "superbeat" (a nickname mixing super classic and downbeat) you will find various threads discussing these. Thanks to Andrew (Fayray) for posting this particular link (in a different thread).
The 55/56 has the 14x24 and 14x6 1/2 option on the Rich super classic.
Maybe we should amend the site motto to "ask a question, get a relevant answer"? :)