Former series (1957-1977) Diplomatic plates where blue on white.  The 1970 plate was undated and validated with stickers for 1971 and 1972.

Former series 1981-1994(?). Diplomatic vehicles had black on yellow plates with NICARAGUA LIBRE at the top and CENTRO AMERICA at the bottom. The registration was of a two digit embassy code (details unknown), CC, CD or MI vertically and two more numerals.
MI = International Missions