snorkel box
n.— «It’s tough to drive up and mail a letter when there is no drive-up letter box. The snorkel boxes, as the Postal Service calls them, have stood at Third and Union for decades.» —“Tunnel Vision Curbs Downtown Drive-Up Mailboxes” by Rick Anderson Seattle Times (Wash.) Jan. 23, 1987. (source: Double-Tongued Dictionary)
