Provenance puzzle #1 -- solved!
A hearty thanks to Stephen Ferguson, Curator of Rare Books at the Princeton University Library, for providing the answer to my Provenance puzzle #1. The stamp is a portrait of Augustus, Elector of Saxony (1526-1586). Stephen used Google Books to find a reference to the stamp in Konrad Haebler's Rollen- und plattenstempel des XVI. jahrhunderts (Leipzig: O. Harrassowitz, 1928-1929), vol. 2, pp. 79-81.
See the Wikipedia article on Augustus of Saxony, where you will learn that Augustus, a Lutheran, played an important and influential role as a peacemaker in the religious conflicts of the early German Reformation.
The stamp is on the front cover of our copy of Practica eximia atque omnium aliarum praestantissima by Giovanni Pietro Ferrari (Frankfurt: Sigmund Feyerabend, 1581), part of the Roman-Canon Law Collection of the Association of the Bar of the City of New York.
Additional images of the covers are in my Flickr gallery in the "Provenance markings" set.
Finally, check out Stephen Ferguson's excellent blog, Rare Book Collections @ Princeton, a favorite of mine.
Rare Book Librarian