I found out that the "3rd Maryland Veteran battalion" captured the 51st VA's flag.

Source:

http://books.google.com/books?id=MBPE0Et_h3kC&pg=PA325&lpg=PA325&dq=57th+North+Carolina+captured+the+57th+Massachusetts+flag&source=bl&ots=dGWv7peRqA&sig=vm2OP_8Jb_f9faZ7F6zQWDu8z2k&hl=en&ei=_7OLSojmAYPkNeuC0MIP&sa=X&oi=book_result&ct=result&resnum=3#v=onepage&q=&f=false

Page 324. Also reports other CSA flags captured at Fort Steadman.

12th GA Battalion lost their flag to the 3rd MD Vet Battalion.
57th Massachusetts captured the 57th North Carolina's flag.
100th PA, captured the 5th VA's flag.
100th PA, captured the 31st GA's flag.
100th PA, captured the 1st VA's flag.

Hope that helps!

Shawn Prouty