The 16 berry reverse (D2 reverse) refers to 1921-P Morgan Dollars with a reverse type that does not include an "extra berry" on the right side of the wreath, which is found on some 21-P reverses that are also known as the 17 berry (D1 reverse) reverse types.

To see the distinction, examine the 17 berry photo first, then the 16 berry photo below it.

17 berry reverse:
1921-P_VAM-1i_extra_berry.jpg
1921-P_VAM-1i_extra_berry.jpg


Compare to 16 berry reverse:
1921-P_VAM-3_no_extra_berry.jpg
1921-P_VAM-3_no_extra_berry.jpg


There are other ways to distinguish the two reverses, and you can find more detailed info on the 17 berry page.

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