Is actually CN1986, a typo? Anyway, it refers to the MOD number and the year of production is there 1954.
1953 brass Britishmilitary whistle
British military whistle. 3 1/4" brass. Stamp-marked with the Ministry of Defence broad arrow and J. HUDSON & CO / BIRMINGHAM / C.N. 1986 / 1953. C.N. 1986 refers to the MOD department and 1953 is the year of manufacture -- the most important year of the 20th Century.