The gold key used to unlock the heavy front doors at the opening ceremony of Old Parliament House has returned home for the first time to mark the 90th anniversary of the historic ceremony.

But mystery surrounds two other keys believed to have been made at the same time.

"Three keys were commissioned," said Museum of Australian Democracy (MOAD) curator Stephanie Pfennigwerth.

"However we can only find one of them — [we] don't know what happened to the other two."

The gold key used to open Parliament House on May 9, 1927. ( Supplied: Parliament House Art Collection )

The 14-carat gold key was designed by Edwin Henderson, a senior architect with the then Department of Works and Railways, who also designed Manuka Pool and Ainslie Primary School.

It was used by the Duke of York, later King George VI, at the official opening of what was then the provisional Parliament House on May 9, 1927.

According to Ms Pfennigwerth, the Duke created "a slight kerfuffle" by handing the key to then prime minster Stanley Bruce after he unlocked the wooden and glass doors.

"A former speaker of the House of Representatives was very upset by this gesture because he interpreted that as elevating the executive above the custodians of Parliament who are the speaker… and the president of the Senate," she said.

Key on display in new exhibition

The key was kept for years in a brown velvet box in a cupboard at the National Library before eventually finding its way to the custodianship of the speaker.

It is on display for six weeks at The Opening Day: 9 May 1927 exhibition which tells the stories of the people who witnessed the ceremony.

Ms Pfennigwerth said the key symbolised the importance of the event, that was attended by dignitaries, politicians and residents of the fledgling city of Canberra.

"Having a new place for the new Parliament in a new national capital of a new nation was… a highly symbolic moment," she said.

"It consolidated people's ideas of nation and identity."