The small metal plate usually on the centreboard case would have been issued by the copyright holder (Ian Proctor Designs) as an indication that he license fee had been paid by the builder. This plate would have the sail number stamped on it.
Some builders may also put their own plate on as well.
The metal plate is a pretty good mark that it is a bona fide Wayfarer.
Lack of a metal plate does not necessarily mean it is not a bona fide boat – just more difficult to check.
If the current owner has a measurement certificate then that is also a very positive sign.
Good sources of used boats can be found on the classifieds;
There is also a link to Ian Porter Sales – a recommended source of used boats.
Hope that helps