What information must signatories provide when signing the proposal?

Ley Orgánica 3/1984, de 26 de marzo, Reguladora de la IniciativaLegislativa Popular.

http://www.boe.es/buscar/act.php?id=BOE-A-1984-7249 

1. Besides the elector’s signature there should be writtendown his/her name, the National Identity Card number, and the municipality inwhich electoral lists is registered.

2. The signature must be authenticated bya Notary public, by a Judiciary Secretary or by the municipal Secretary thatworks at the municipality where the elector that has signed is registered. Thisauthentication must record the date when it has been done and it could be acolective authentication, page by page. In this case, the number of signaturescontained in each page must be written down besides the date of theauthentication.