posted some infomation about some of the latest Stargate prop work auctions here.
Included in the auction was Sheppard's dog-tags from season one and here is the tidbit she posted about it::
Technical description: Set of two production-made metal military dog tags with black rubber silencers on two bead chains with clasps, both tags are imprinted with "MAJOR SHEPPARD 163 23 4111 AB POS RC",
both tags measure approx. 2in x 1.25in. Chains measure approx. 24in and 4in long.
If the tags were real, it should have read "John Sheppard, Major." Sheppard's blood type is AB positive and he's Roman Catholic. That social security id number was issued in Pennsylvania. While you used to be able to determine the state the SS# was issued by its first 3 digits, starting in June 2011, the numbers will be randomized to help prevent id theft.
ETA: from watcher652
eta: according to the link provided by [info]velocitygrass in the comments, the name should read Sheppard, John (middle initial, if any). I had just repeated someone else research without verifying it (bad on me). I thought it was weird with his rank on his tags. That would have meant new tags for every promotion. Also, the fact that Sheppard is Air Force would have been noted with "AF" after his id.
Pretty cool little details for authors out there.