According to the two Makers Marks websites I use ...
D.O.C. is for the Dominick O. Cunningham Glass Company of Pittsburg, Pennsylvania who was in operation between 1880 and 1931. So I'd say your bottle is at least pre-1932
Josh
I tried sending this through a PM but couldn't. If you haven't already seen it, you might want to check it out ...
http://www.ebay.com/itm/Eppings-7UP...574768?hash=item25b3c13970:g:nOUAAOSw3KFWfyos