The WW2 vintage dog tag was different from the more modern tag.
Two tags were issued to each man.One carried his name, serial number, blood type, religion and dates of Tetanus shots.
The second tag had the man's serial number and the name and address of his next of kin.This tag also had his religious preference.
The WW2 type, while the same basic shape as the modern tag, had a small notch formed into one corner.Popular lore has it that this notch was intended to hook the tag between the front teeth in the event that the man was a combat casualty, and Graves Registration would look for the tag there if the man was killed in combat.The explanation for this notch given by Army Mortuary Services today is that this notch was intended to allign the tag in a machine when the tag was used to process paperwork.The tag acted much like a credit card and transferred the personal data to a paper form much like an older type credit card imprinter.This saved time in having to type the information several times, and reduced the possibility of error, as it transferred the exact data to the paper forms.Apparently the tag holder had a small post which alligned the chain hole of the tag, and the notch fitted into a corresponding groove on the opposite end securing the tag against lateral movement.
I have an acquaintance who recently acquired a WW2 vintage dog tag machine andhas contracted for reproductions of the original blank tags.I will forward his email to you.
Navy and Marine Corps tags were different yet again.They were more rounded than losenge shaped, but still had the slightly rectangular shape.Perhaps oval-shaped is more accurate.While they contained the same basic information, the reverse of the Navy tag had a small square area that was chemically treated.At processing the Navy and Marine personnel would put their finger on a chemically treated pad and put their fingerprint on the treated square of the tag.
Tags were made from several different metals including stainless steel, and brass alloy.Colored tags of a later vintage indicated special medical information such as allergies to medication, but I am not sufficiently aware of the differences to discuss them.
When you create the tag for the next-of-kin be sure to utilize the 1940s version of state abbreviations (like Iowa, Wisc, Mass...) and, if from a large city, the "area code" of the city, such as St. Louis 6 Missouri.City Registers in libraries of the subject city will have the area breakdowns for reference.