I've seen Corsa manufactured by both Carnielli and Rizzato. Salvatore Corso was an importer for both Atala and Bottecchia, so it's not surprising to see Corso bicycles from both Rizzato and Carnielli. Given the apparent era, if it's Rizzato, manufacture, it will have an open format serial number on the upper seat tube while a Carnielli built frame will have the serial number on the underside of the bottom bracket shell.