"Becco" refers to the nozzle, and the "volute" are scroll-shaped decorative features (volutes) on either side of the nozzle. You could also phrase it as "ogival nozzle flanked by volutes", but "voluted" makes it more concise.

"This circular lamp has a narrow, voluted nozzle extending from it..."
http://www.medievalportland.pdx.edu/?q=round-terracotta-lamp...

"Bronze lamp on a high circular base, with a broad round body and an ogival nozzle with a double volute..."
http://issuu.com/phoenixancientart/docs/v1.3web_ctlg265x265m...

You can make out the volutes on the nozzle in this picture, just to the left of the spout:
http://www.metmuseum.org/collection/the-collection-online/se...|Lamps&pos=14