I get all misty eyed when London allocate a 4321 as an ORCAM squawk! I have got the original drawing that illustrated an article in Flyer magazine. The drawing is of parrots in 'plums' of the colours of the European countries (Squawking 7000 & 7001) and the UK Parrot squawking 4321. Also kept a copy of the original magazine article as well – when I find it (just moved house) I'll post a reply!
Found them - just a quick picture!
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The flyer article states the change will occur on the 5th March that year. The accompanying articles states preceding years 1990 & 1991. Therefore I conclude that the change took place on 5 March 1992. This would appear to be supported by the article here :
level douglas | twin otter | capt speaking | 1992 | 0270 | Flight Archive which is dated February 1992.