Locating a "libro reservado" in baptismal records, Asuncion de Maria, Aguascalientes

In reviewing the digital copy of an indexed baptism, the priest indicated that the names of the parents and the grandparents paternos and maternos were located in the "libro reservado," Checking several pages in front of and behind this record, I could not locate another record in which the priest referred to the "libro reservado." This record was indexed and the indexer listed the names of the parents and the grandarents, which I have located in other records,however there is a discrepancy with the surname of the maternal grandmother.

"México, Aguascalientes, registros parroquiales, 1620-1962," database with images, FamilySearch (https://familysearch.org/pal:/MM9.3.1/TH-266-11811-102828-20?cc=1502404&wc=M6QX-76D:64894501,64894502,65547701 : accessed 18 September 2015), Aguascalientes > Asunción de María > Bautismos 1861-1862 > image 470 of 504; parroquias Católicas, Aguascalientes (Catholic Church parishes, Aguascalientes).

If, as the record is indexed, Jose Pedro is the son of Franciso Luebano and Cleofas Prieto, then his marriage record indicates that he was the hijo legtm of Francisco Luebano y Cleofas Prieto, so I don't think I am looking for a book of baptisms for "hijos naturals."

I'm hoping that someone can direct me to the location of the "libro researvdo," and offer an understanding of this term.

Thank you,