View link at bottom of this post for your one stop shop website for positive ID of your collar disc. Yours is made between 1910-1924.
The back of yours would have looked just like this....
http://hglanham.tripod.com/metalinsignia/attdisk1.jpg
http://www.snyderstreasures.com/eBay/Images/2010/031710/DSCN0228.JPG
USDUI-164
US World War One Air Service Signal Corps EM Collar Insignia Matched Pair
The last pic was found on this site....http://www.snyderstreasures.com/pages/usmilitaria.htm
And here are Locket Discs that open up, see if yours is hollow....http://www.usmilitariaforum.com/forums/lofiversion/index.php/t142001.html
http://i175.photobucket.com/albums/w142/oznola45/Lcktdsc-3.jpg
http://i175.photobucket.com/albums/w142/oznola45/Lcktdsc-4.jpg
This forum explains that these collar discs were very short lived, from 1918 to 1920.
http://www.usmilitariaforum.com/forums/lofiversion/index.php/t2695.html
What it looked like on the collar...
http://quanonline.com/military/military_reference/american/wwi_uniforms/pics/wwi_mc_collar3.jpg http://quanonline.com/military/military_reference/american/wwi_uniforms/pics/wwi_mc_tunic.jpg
Explaination of the WWI uniform and these collar discs.....http://quanonline.com/military/military_reference/american/wwi_uniforms/wwi_mc_uniform.html
Finally, your one stop shop for collar disc ID'ing http://hglanham.tripod.com/metalinsignia/collardisk1.html