My note;
When the 4th Marine Brigade became part of the 2nd Infantry Divison US Army to simplify things the numbers were changed to the letters below;

FIFTH REGIMENT.
First Battalion.

17th (A) Company.
49th (B) Company.
66th (C) Company.
67th (D) Company.

Second Battilion

18th (E) Company.
43rd (F) Company.
51st (G) Company.
55th (H) Company.

Third Battalion.

16th (I) Company.
20th (K) Company.
45th (L) Company.
47th (M) Company.

8th Machine Gun Company.
Supply Company.
Headquarters Company.

SIXTH REGIMENT.
First Battalion.
74th (A) Company.
75th (B) Company.
76th (C) Company.
95th (D) Company.

Second Battalion.

78th (E) Company.
79th (F) Company.
80th (G) Company.
96th (H) Company.

Third Battalion.

82nd (I) Company.
83rd (K) Company.
84th (L) Company.
97th (M) Company.

73d Machine Gun Company.
Supply Company.
Headquarters Company.

SIXTH MACHINE GUN BATTALION.
15th (A) Company.
23d (B) Company.
77th (C) Company.
81st (D) Company.

http://www.maxwell.af.mil/au/awc/aw...ist/2nd1970.txt
This is a good read on the 2nd Regiment pior to 1901.
Now reading again, I come away thinking this is a mix bag.
He sites A, B, C companies at the begining...than he sites On 26 March, the 36th, 100th, 148th, and 153d Companies joined the regiment.
On 17 December, the 36th, 57th, 63d, 100th, 148th, and 196th Companies were transferred to the 8th Regiment. With the organization of the Supply Company on 24 January 1920, the 2d Regiment was composed of Field and Staff, Supply, 53d, 54th, 62d, 64th, 153d, and 197th Companies.
He also states that the numbers were still in use up to 1924.
So the change from numbers to letters must have been slow in coming but it happen pior to WW II.
Many books on WW I and the part of the 5th and 6th Marine Regiments part of the 4th Marine Brigade.
State just the Regiments, no companies but these after action reports shows the different companies.
I hope that this would be part of History and Traditons of the Marine Corps.
Because the members of those companies made additions to the glorious history of the Marine Corps

"Retreat Hell, We just got here!"

__________________

"A man or woman is measured
by the footprint,
he or she leaves behind".

"They were the best you had, America,
and you turned your back on them".
~ Joe Galloway ~ Speaking about Vietnam Veterans

TO THOSE WHO GAVE THEIR LIVES
THAT OTHERS MAY SAY PROUDLY
I AM A MARINE

Semper Fidelis/Semper Fi

Ricardo