Not a stamp, but rather a cancel.
Per Bruce Baryla, from his award-winning "Civil War Sun Tax" exhibit:This 'Skull and Bones' handstamp was known to collectors of fancy handstamps by its appearance as a 'sender's mark' on covers mailed in 1857 by the 'Society of Twenty-Two,' a Yale University based fraternal group associated with the famous Skull and Bones Society.
This same handstamp was used years later as a stamp canceling device. As it turns out, the secretary of the 'Society of Twenty-Two' in 1857 was Robert A. Beckwith. He became a photographer and co-owner of the Whitney & Beckwith studio - and he repurposed the handstamp from his college days to cancel revenue stamps.