The most common air raid whistle found is the J. Hudson whistle that has "A.R.P." engraved on it. Also manufactured during the war was the Adie Bros. (Brothers) version that appeared in 1941, identical in design to the Hudson one. This company, also based in Birmingham, had previous government contracts for whistles and many appeared with the Ministry of Defence broad arrow - crows foot mark /|\ - and some with a year date.
In 1941 Adie Bros. received a contract for 40,000 ARP whistles (previously it appears that just Hudson has been manufacturing ARP whistles). Hudson's received a contract at the same time for 60,000. The reason for so many was that a change in how the alarm for the fall of incendiary bombs was to be made. Short blasts would indicate incendiaries. At this time whistles were now issued to all reserve wardens and fire guards/ fire watchers. Oddly, Supplementary Fire Parties did not get them. The Adie ARP whistle features the maker's name (ADIE BROS), city (BIRMINGHAM), royal cipher (GR VI) and year (1941). The 1941 ARP whistle omitted the MoD broad arrow as the contract came through the Ministry of Works & Buildings.
Read more about the history of ARP whistles