We are grateful to Prof. David R. Marshall for identifying the sources for these views and providing information for cataloguing the pair (written communication, 7 November 2008).
These views are based on two prints designed by Giuseppe Galli Bibiena (Parma 1695-1757 Berlin) and engraved by Andreas Pfeiffel in Architetture propettive dedicate al Maestà di Carlo Sesto, Imperador de' Romani da Giuseppe Galli Bibiena, sup prmo ingegner teatrale, ed architetto, inventore delle medesime, published under the direction of Andrea Pfeiffel, Vienna, in 1740. Bibiena's engravings are Venetian in conception and would also serve as the model for several plates in Piranesi's Prima parte of 1743.
The round temple with the portico in the view on the top is taken from plates Part III, no. 6. The temple in the other view repeats the Mausoleum of Hadrian (Castel Sant'Angelo) as depicted in Part III, no. 10, which Bibiena, in turn, took from Fischer von Erlach's views of the Pons Aelius and Castel Sant'Angelo (in Entwurff einer historischen Architectur, 1725). A preparatory drawing attributed to Giuseppe Galli Bibiena for the latter is in the Policlinico, Turin.
The artist at work here is either a Venetian follower of Michele Marieschi or, as Prof. Marshall suggests citing the Austrian print source, an artist working in the circle of Joseph Platzer.