I've own this coin for years and I found it roll searching quarters . I just weight it and it weights 5.7 grams . It should weight 6.22 grams, that's a lot of under-weight . To me it look like its on a copper planchet and not a bronze planchet . Help ..........
due to weight tolerance the two cent piece could weigh as low as 5.96 grams, add the heavy wear on the coin and 5.7 is probably not out of line.
Here's a new image after its been in a hard arcic case for 10 years or so . Here's a link I want everyone to check out, maybe its on one of theses planchet ( 1863 312a ) http://coinauctionshelp.com/Pattern_Guides/twocentshield.html#.VbgLY7Ul_Sh
I can't match your coin up with any of the pattern coins on that link. Let's see what they say over at CU.
312a? I see significant differences especially on the reverse. The best way to determine the planchet composition would be to have it tested. Kinda expensive but it would answer your question.
Heres a scale to change grains to grams and it fits within the limits http://www.metric-conversions.org/weight/grains-to-grams-table.htm