Headon is the Centenary of Canberra history and heritage adviser and the object of his reverent unwrappings was a golden cigarette case. What makes it so very special a treasure is that it is the very case which, on the day of our city's naming, March 12, 1913, contained that hitherto secret and mystical word chosen to be our city's name. Lady Denman took the piece of paper on which it was written out of the case and then spoke that mystical word, announcing the federal capital city's name to the waiting crowd and the waiting world.

When Headon calls it the ''greatest treasure of them all'' he means that it is the greatest of the several centenary-precious treasures of the Denmans that he and others have been able to find and bring to Canberra to display. Other treasures include the fabulous cocked hat, and its plumes, that governor general Lord Denman wore on the great day. (Alas, Lady Denman's even more fabulous hat of the day, extravagantly bedecked with ostrich feathers, has been lost without trace.) These and other wonders (including Lady Denman's cream silk parasol with ivory handle) will grace the But Once in a History exhibition that opens at Parliament House on Monday.

It is impossible to think of a more ''sacred'' item, sacred to this year's centenary of naming of the city, than the little cigarette case. That it should have been found and that it should have been delivered to Australia (''thoroughly insured by the ACT government'' a delighted Headon chortled) to be shown in the very city it is so vital to, seems almost miraculous. Headon explained on Tuesday that it had been thought lost but that his and others' assiduous detective work and the terrific co-operation of Denman descendants had borne fruits that included the cigarette case and the cocked hat and other gems.

Lady Denman had been given the cigarette case as a present, a memento of the March 12, 1913, occasion, and it had been used then and there on the day as a little golden casket to contain the chosen name.

It was an apt present because, Headon reminded us, unorthodox Gertrude Lady Denman was that rare thing for a woman of her class, a smoker. And a heavy smoker too, Headon fancies, for the cigarette case, though elegant (it has the Denman family crest on it and is inscribed within) shows some humanising signs of considerable use.

The case is indisputably the greatest Denman treasure but Lord Denman's hat (the word hat doesn't do justice to this masterpiece of the art of a distinguished Piccadilly milliner, festooned with lots of silver braid and accompanied by plumes) is wondrous too. It comes in its very own inevitably hat-shaped metal container, grandly decorated with a brass plaque proclaiming to whom it belongs.

Lord Denman was slightly built (about 168 centimetres) and the hat is petite. A boy could wear it.

And, Headon delighted in telling us, boys have worn it because in conversation with a Lord Denman descendant, he, Headon, was told that boys of the family used to go up to the attic of the family castle in Sussex and get out their lordly forebear's items, including the hat, and dress up in them.

Lady Denman's cigarette case reminds us that, a good-hearted soul and sensitive to smokers and their needs, back in England after the vice-regal sojourn in the antipodes, she threw herself into war work (for the Great War of 1914-1918). That work included collecting cigarettes and tobacco for the boys at the Front.

Her admiring biographer tell us that ''Trudie [an affectionate shortening of Gertrude] became the moving spirit in a war charity called the 'Smokes for Wounded Soldiers and Sailors Society'. ''

''The Society [the S.S.S.] operated from 4 Buckingham Gate [the Denmans' London home] where Trudie turned the ballroom into a packing room. The house was soon crammed with cigarettes and packers. The S.S.S. voluntary workers met all hospital ships and trains … by the time she resigned in 1917 under pressure of other war work 265 million cigarettes had been distributed as well as large quantities of tobacco, pipes and cigars.''

>> The exhibition But Once in a History, curated by David Headon and all about ''Canberra's grand foundation narrative'', opens at Parliament House on Monday.