There exist a monograph ("The Disinfected Mail of Spain" Postal History Society, 1972) about the disinfection of mail in Spain by Sidney Nathan. In this booklet Nathan stated "Slitting of letters was not carried out until 1800, at least no example of earlier slitting has so far been discovered, ...)".
The earliest slitted letter from the Spanish mainland I have in my collection is from Catalonia and dated Aug. 1st, 1798.
The book "Manual de las Tarifas Postales de Espana y sus Territorios de Ultramar" by Gracia Herráis, J. A. shows on page 142 a letter from Cádiz to Mahón with four diagonal and one central slit which was written on Oct. 4th, 1797 and was disinfected at the Mahón lazaret (Menorca).
Are there any earlier slitted Spanish covers recorded?