Take your time and have a look around! Look for Sarg around the web site with helpful hints.
There are many explanations for the Notch on the WWII / Korean Conflict dog tags. The notch refers to the "V" cut out on the short side of the dog tag across from the hole.
The most popular explanation for the notch is to place the tag securely in the teeth on a fallen soldier's body. This may have been done but that was not the original purpose of the notch. There's a great story told about a soldier who was kicking the tag into the teeth of a "dead" soldier and actually "woke" him up!
Another popular belief is that the notch was used in the old Graphotype dog tag stamping machines in order to line up a blank dog tag for proper tag alignment during tag stamping. We have used both electric and hand crank machines from the WWII era, and have never figured out how a notch would help in the alignment of the tag.
The best explanation is the notch's use by the Medical Corp submitted by Alan Batens.
"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 (see above graphic). 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".
The red arrows in the picture above, are showing the carbon marks from the use of the Addressograph to transfer data from a soldier's dog tags to the "Report of Burial".
Many thanks to Alan S Batens for providing the above explanation and the following pictures. Be sure to check out his very thorough web site, The Combat Medic at: http://home.att.net/~steinert/
Today, the notched tag is no longer used by the US government and has been replaced by one tag placed on a short chain, 5.5" long, commonly referred to as the "Toe Tag".