longstanding though unauthorized or irregular work practices.
This expression has been in use in printing circles since the 1960s. It is often used humorously to refer to practices in the British newspaper printing houses in Fleet Street – London…formerly notorious for their inefficiency. The reason for describing such practices as Spanish is not known.
1998 - Spectator - [Outsourcing] can do much for flexibility and more for costs and it is a proven cure for quaint old Spanish customs.