By Bethany Bell
BBC News, Salzburg
Pianist Florian Birsak plays one of the newly-identified pieces
The Tanzmeistersaal in the Mozart Residence Museum was full, and there was a keen sense of anticipation in the air.
After all, it is not every day you get to go to a premiere of pieces by Mozart - played on his own piano and in his own house.
Posthumous discoveries of works by Wolfgang Amadeus Mozart are rare but not unknown.
And the two short pieces unveiled in Salzburg appear to be a "missing link" in the young composer's development, according to Dr Ulrich Leisinger from the International Mozarteum Foundation.
The first piece to be performed was the Concerto in G (molto allegro) - probably the first movement of a harpsichord concerto written in 1763 or 1764, when the composer was around eight years old.
Only the solo part of the harpsichord was written down.
Researchers at the Mozarteum believe it forms an important link between the miniatures Mozart wrote as a very young child and the larger instrumental pieces he went on to compose later.
The Harvard professor Robert Levin says: "What the composer expects of the player in racing passagework, crossed hands and wild leaps is more than a bit crazy.
"I consider it quite credible that the movement was composed by the young Mozart, who wished to show in it everything he could do."
There are anecdotes which suggest that Mozart began to compose concertos long before his first "official" piano concerto, K 175, in 1773.
The Salzburg court trumpeter and close friend of the Mozart family, Johann Andreas Schachtner, described being shown an inkblot-stained score of a part of concerto written by the young Mozart.
Mozart's father, Leopold, had at first dismissed the piece - but then looked at it a little more closely.
"Look here, Mr Schachtner," he said. "See how everything is correct and regularly set - it is only useless because it is too difficult for anyone to play."
The young Wolfgang was not abashed. "That's why it is a concerto," he said. "You have to practise a long time before you can play the notes. Here's how to do it."
The second piece, the Prelude in G major, is also technically demanding, but described by researchers as slightly more "refined".
It was the "crazy" and virtuosic nature of the pieces that helped the researchers at the Mozarteum identify them as being by the young Mozart.
It is thought Mozart's father Leopold transcribed the pieces
The works were part of "Nannerl's Music Book", a collection of music compiled by Leopold Mozart, in the archive of the International Mozarteum Foundation since 1864.
They are written in Leopold's handwriting - but Dr Leisinger believes he transcribed pieces his son played on the piano.
"This was a young composer running riot to show what he was capable of," Dr Leisinger said.
"The piece does contain real technical mistakes and clumsy moments that an old hand like Leopold Mozart would never have made."
The Austrian musician Florian Birsak played both pieces on Mozart's fortepiano - and then a short orchestral version of the concerto was performed.
The missing orchestral accompaniment was written by Robert Levin, who specialises in historical performance.
There will be another performance of the pieces during Mozart Week 2010 in Salzburg.