Note that in the British Army, the plural was "platoon sergeant majors" and not "platoon sergeants major". "No. 48587". The London Gazette (Supplement). 16 April 1981. p. 5671. "No. 25044". The London Gazette. 2 December 1881. p. 6467. The earliest usage of "sergeant majors" in The Times is in 1822. The last of the (very occasional) usages of "sergeants major", except when referring to American NCOs, is in 1938.
