FIRST, A BIT OF TRIVIA:

Throughout WW2, the US military utilized the M1940 dog tag. This is the one with a notch on one side. Rumors have always speculated that the notch was used so the tag would remain in the dead soldiers teeth, but that was not the intended purpose, although it was probably used for that.

Another common rumor is the notch was used to properly align the tag so it could be embossed.  If you have ever seen or used a WWII style embosser, you will find the notch offers no practical way of improving alignment.

The best explanation I have seen comes from ArmyDogTags.com. Their explanation is as follows:

“The Model 70 "Addressograph" was a pistol-type imprinting machine used by the Medical Department during WWII. Its function was to transfer the wounded soldier’s identification information directly from his dog tags to his medical records. The notch in the dog tag would align and hold the tag securely in the "Addressograph". First the dog tag was inserted into the imprinting machine. After the medical document was aligned in the "Addressograph", the trigger on the imprinting machine was pulled and the information on the dog tag was transferred to the medical document through the ribbon of carbon paper located inside the "Addressograph".”

DETERMINING AN ARMY SERIAL NUMBER
First Fragment or leading digit of the serial number
1: Volunteer, Regular Army (8 numbers)
2: Reservist/National Guard (8 numbers)
3: Draftees (8 numbers)
O-:Commissioned Officers (2-7 numbers)
N: Nurse Corps (6 numbers)
T: Flight Officers (Variable)
S: Specialist Corps (Variable)
L: WAC Officer (6 digits)
A: WAC Soldier (6 digits)
V: WAC Warrant Officer (6 digits)
--There are more, but these are the majority of the common ones issued.
Second Fragment
01: Hawaii Department
02: Panama Canal Department
03: Philippine Department
04: Puerto Rico Department
1: Maine, New Hampshire, Vermont, Massachusetts
1: Rhode Island, Connecticut
2: New Jersey, Delaware, New York
3: Pennsylvania, Maryland, Virginia, Washington D.C.
4: North Carolina, South Carolina, Georgia, Florida
4: Alabama, Tennessee, Mississippi, Louisiana
5: Ohio, West Virginia, Indiana, Kentucky
6: Illinois, Michigan, Wisconsin
7: Missouri, Kansas, Nebraska, Iowa, Minnesota
7: Wyoming, North Dakota, South Dakota
8: Texas, Oklahoma, Colorado, New Mexico, Arizona
9: Washington, Oregon, California, Idaho, Montana
9: Nevada, Utah, Alaska (Attached)
 *  Men of the same surname will be assigned consecutive ASN numbers. (Brothers, Sons, Fathers, etc)
 *  The number for Corps (Second Fragment) would be the area of enlistment.

 Tetanus Dates
The dates on line 2 of the styles indicate the tetanus immunization, and the tetanus toxoid injection dates. These would most often be 1 year apart, and preceded by a T for the first date (i.e. T43 44).

Blood Type
The military categorized blood according to four most basic forms of blood. They are: A, B, AB, and O. The RH factor wasn’t available until 1940 but the US military did not use it during WWII.

Next of Kin Address
Through the early part of the war, the next of kin to be notified was printed right on the dog tag, but this was later removed for security purposes after the tags changed in July 1943. Furthermore, the two letter state abbreviation was not standardized, so standard abbreviations were sometimes longer. 1942-1943 do show some shortened abbreviations, and if nothing else, shorten even further when approaching the 17 letters allotted (remembering that from 1941-1943 the standard form had a letter for religion).

Standard abbreviations are as follow:
State - (Common WW2 Abbreviation) - Modern Abbreviation
Alaska - (None) - WV
Alabama - (Ala.) - AL
Arkansas - (Ark.) - AR
Arizona - (Ariz.) - AZ
California - (Calif.) - CA
Colorado - (Colo.) - CO
Connecticut - (Conn.) - CT
Washington, D.C - (None)
Delaware - (Del.) -DE
Florida - (Fla.) - FL
Georgia - (Ga.) - GA
Hawaii - (None) - HI
Iowa - (None) - IA
Idaho - (None) - ID
Illinois - (Ill.) - IL
Indiana - (Ind.) - IN
Kansas -(Kan.) - KS
Kentucky - (Ky.) - KY
Louisiana - (La.) - LA
Massachusetts - (Mass.) - MA
Maine - (None) - ME
Maryland - (Md.) - MD
Michigan - (Mich.) - MI
Minnesota - (Minn.) - MN
Missouri - (Mo.) - MO
Mississippi - (Miss.) - MS
Montana - (Mont.) - MT
North Carolina - (N.C.) - NC
North Dakota - (N.D.) - ND
Nebraska - (Neb.) - NE
New Hampshire - (N.H.) - NH
New Jersey - (N.J.) - NJ
New Mexico - (N.M.) - NM
Nevada - (Nev.) - NV
New York - (N.Y.) - NY
Ohio - (None) - OH
Oklahoma - (Okla.) - OK
Oregon - (Ore.) - OR
Pennsylvania - (Pa.) - PA
Rhode Island - (R.I.) - RI
South Carolina - (S.C.) - SC
South Dakota - (S.D.) - SD
Tennessee - (Tenn.) - TN
Texas - (None) - TX
Utah - (None) - UT
Vermont - (Vt.) - VT
Virginia - (Va.) - VA
Washington - (Wash.) - WA
Wisconsin - (Wis.) - WI
West Virginia - (W.Va.) - WV
Wyoming - (Wyo.) - WY

Religion

C:  Catholic
P:  Protestant
H:  Hebrew
*    If no religious preference, no letter was printed.

DOG TAG FORMATS

Dec. 1940 - Nov. 1941
Brass Notched Dog Tags
M1940 Dog Tag Chain
First name, Middle Initial, Last Name
Army Serial Number, Blood Type (Right Justified)
First Name (Next of Kin), Last Name (Next of Kin)
Street Address (Next of Kin)
Next of Kin City, State (Next of Kin) 

Nov. 1941-July 1943
Monel Notched Dog Tags
M1940 Dog Tag Chain
First Name, Middle Initial, Last Name
Army Serial Number, Tetanus Immunization, Tetanus Toxoid, Blood Type
First Name (Next of Kin), Last Name (Next of Kin)
Street Address (Next of Kin)
Next of Kin City, State (Next of Kin), Religious preference (Right Justified)

 July 1943-March 1944
Steel Notched Dog Tags
M1940 Dog Tag Chain or Beaded Dog Tag Chain
First Name, Middle Initial, Last Name
Army Serial Number, Tetanus Immunization, Tetanus Toxoid, Blood Type
(Blank)
(Blank)
Religious Preference (Right Justified)
 

March 1944-April 1946
Steel Notched Dog Tags
Beaded Dog Tag Chain
Last Name, First Name, Middle Initial
Army Serial Number, Tetanus Immunization, Tetanus Toxoid, Blood Type (Right Justified)
(Blank)
(Blank)
Religious Preferences (Right Justified)