The pre-AIA law granted a one-year grace period from when the invention became known in certain ways to when an inventor had to file their patent application.
To effectively swear back of a reference, an inventor submits a declaration to the US patent office, with written evidence that shows they fully conceived of the invention before the effective date of the reference.
They must also show they were diligent in either reducing the invention to practice or in filing a patent application.
Documentary evidence must support all factual assertions in the swear back.
A copy of a notebook page that describes the invention, signed by the inventor, dated, and preferably witnessed by a third party can serve as written evidence.