The 20ga on GI is one of the models (EGL) specifically referenced in Zutz's section on Vouzelaud as being a sideplated boxlock. Dr. Nelson, who visited Vouzelaud sometime prior to 1992, also confirmed that all their guns were boxlocks.
The Gunbroker gun certainly should be a sidelock with the "platines" notation (although sideplated boxlocks are referred to as "fausses platines"). The GP that's part of the serial number makes me wonder if it's a later model--the P possibly standing for "platines"--by which time Vouzelaud might have been producing at least some sidelocks. Since their guns were/are made by St. Etienne outworkers, it's not like they had to convert a factory to making a new design.
