Very many years ago I got a little bundle of docs from the USMC Historical branch, among which was "A brief history of the Squad in the Marine Corps" dated 1 Sep 1955.
It really only does refer to the Rifle Squad, and gives a few details for salient dates.
1919 – Squad consists of 1 corporal, 6 riflemen and 1 auto-rifleman. One rifleman is equipped with a grenade discharger. One rifleman carries ammunition for AR and serves as replacement AR man.
1922 – USMC issues first T/Os, kept eight-man Squad with single BAR.
1923 – Squad as above but allowed for two riflemen to act as scouts.
1933 – 4th Mar Regt (China) and 1st Mar Bde (Haiti) began to experiment in using an eight-man peacetime Squad that was expanded to 13-men in wartime. Eight-man peacetime Squad of Corporal (rifle), four riflemen, one auto-rifleman (BAR), one rifle-grenadier and one man with TSMG. 13-man Squad wartime Squad had Sergeant (leader, with rifle), two Corporals (asst leaders, rifles), six riflemen, two auto-riflemen (BARs), two rifle grenadiers.
1934 – 'new' Squad introduced based on eight-man org as above.
1935? – 6th Mar Regt conduct new trials, using 8, 11 and 13-man Squads. These latter two types allowed for the Squad to subdivide into two equal parts (no mention of fire teams here), with the six-man half of a 13-man Squad carrying the same amount of ammn for its BAR as the current 8-man Squad did. Capt Hannenken (E Co) recommended using the 13-man Squad, handled as two 6-man parts, both under the direction of the Sergeant. Proposals recommended but not actioned pending further developments (arrival of the M1 rifle and an updated BAR).
1936 – Squad confirmed as eight-men with one BAR and one TSMG (which flipped between being the Squad leader's weapon or being carried by a rifleman).
1938 – Squad increased to 9-men. Corporal (rifle), five riflemen, two auto-riflemen (each with BAR) and a rifle-grenadier.
1941 – T/O D1 of 28th Mar 1941 issued. This allowed for the 9-man Squad to be increased to 13-men. Nine men organised as Corporal (rifle), two scouts (rifles), rifleman-grenadier, auto-rifleman (BAR), three riflemen and a rifleman acting as 2-in-C. 13-man model eliminated the single BAR unless M1903s were carried, and had a Sergeant, Corporal and 11 riflemen.
1942 – T/O D1 of 1st July 1942 issued. Squad now Corporal (with SMG), six riflemen, two rifle grenadiers and one auto-rifleman with BAR.
The article doesn't reference the BAR Squad that appears to have come in with D-1, nor give anything on Platoon level.
Gary