Februs:
I know next to nothing about the Spencer Spanking Plan but I would say that one has to be very careful in determining what is actually true from what has been merely passed around.
The plan is basically a domestic contract (with background to tell the newbies what a domestic contract is) that can result in either party being spanked. It was produced as a pamphlet by someone calling herself Dorothy Spencer in 1936 and is confirmed as being copyrighted by that person in a 1941 Catalog of Copyright Entries produced by Library of Congress Copyright Office.
Of course that leaves many unanswered questions:
Was that her real name? Was it really a "this is how to make a relationship work" thing? Was the writer a 50 year man jerking off at the idea of couple buying into the plan and spanking each other? Does the plan actually work?
No, I don't have answers to any of those questions - but in finding the record of the copyright I believe I've disproven the statement: "It simply reads like a document written in the late twentieth century - rather than, as it purports to be, written in an earlier part of that century - to be anything else." - that proclaimed it a hoax.
Goodgulf