The wild animals had been moved out but that didn't stop one 19th century joker trying to pull a gentle April fool, an archive has revealed

The proud recipients of an invitation sent out in 1856 must have felt they were being called to witness a magnificent event that joined history, royalty and a superb setting – British pomp and circumstance at its best.

Signed by one Herbert de Grassen, evidently a "senior warden", sealed with an imposing blob of crimson wax, and with the stern warning "It is requested that no Gratuities will be given to the Wardens on any account", it summoned recipients to witness "the Annual Ceremony of Washing The Lions" at the Tower of London.

The sharp-eyed might just have spotted the date: Monday 1 April. History, alas, does not record how many missed that crucial detail and arrived at the royal menagerie at the tower.

The invitation survives in the archives of the tower and, although the original is too fragile to display, a replica will be on view when a new exhibition opens at the White Tower, the London home of the Royal Armouries museum, on Saturday. It deals with the many institutions that operated from within the tower's massive walls, including the Ordnance Office, the Ordnance Survey, the Royal Mint, the Record Office and the Royal Observatory.

The tower's imperious black ravens are the last survivors of the menagerie which was kept there from at least the 14th century, stocked with many gifts from other countries, including lions, leopards and a polar bear which had a special licence to fish for its supper in the Thames. Earlier in the 19th century – before the date of the invitation – the lions and other animals were moved to the new Regent's Park zoo.