Jürgen Marcussen installed his first organ in 1819 at Sieseby, near Kappeln. In 1968 Marcussen & Son installed a new organ, behind the historic front. The old organparts, of quite special significance for Marcussen & Søn, were preserved, and in 1986 a the re-siting of what may be called the firm’s Opus 1 – but which strictly speaking is Opus 2 – as the choir organ in Haderslev Cathedral, was carried out. This instrument was built in 1819-20 for the church in Sieseby by Slien and is certainly the oldest preserved and the first Marcussen organ of any size, and today probably “the world’s oldest organ from a still-existing firm of organ builders” – which moreover includes several unusual and creative details of a historical nature in organ building. The Sieseby organ could to a large extent be re-sited without any significant changes to the form it had when it was rebuilt by Marcussen & Reuter in 1826. The organfront in Haderslev Cathedral is new-built, on the basis of drawings made by a Danish organ architect, Rolf Graae.

The old photo shows the original organ in Siseby Church.