A Tantalus is a small wooden cabinet containing two or three decanters. Its defining feature is that it has a lock and key. The aim of that is to stop unauthorised people drinking the contents (in particular, "servants and younger sons getting at the whisky"), while still allowing them to be on show. The name is a reference to the unsatisfied temptations of the Greek mythological character Tantalus.
Very few Betjemann examples survive in complete condition; those that do are generally sold at auction for sums in the thousands of US dollars. Original Betjemann articles should have brass or silver plate stamps signifying their authenticity. Later models, in completely different styles, were also called "The Betjemann Tantalus" even though no cabinetry was present and they were not made at the Pentonville works.