Non-Existant Screenwriters - 1950's and 1960's

[ScriptNurse]

Since there is a provision in the Writers Guild of America contract that permits a screenwriter to elect to use a pseudonym if he/she so desires, there hasn't been the use of the fictional name in the same way that directors use Alan Smithee. For fun, here's a source of pseudonyms and their real names:

http://go.to/realnames

[brothertruffle880]

There were movies in the 1950's and 60's which used a generic and fictitious "screenplay by" credit.  This was due to the screenplay being written by a group, or by no one famous.   
Question:  What was this fictitious name?   
Question:  Was the reason they used the fictitious name due to a desire for anonymity?  Any other info given would be appreciated.
Thanks,
Bob