Most of the ones with the breasts covered were before the Civil War. In 1875, the seal exposed both breasts. Of the several versions after that, I believe only hte 1904 version covered both breasts. The current design dates to 1912 and was made official in 1950, although the description in the Code of Virginia (§1-500) makes no mention of Virtus' breast being exposed.

While Virtus has one breast exposed on the obverse, Libertas, Aeternitas, Ceres are fully clothed on the reverse.