BR-Klassik will broadcast a previously unreleased portion from the July 12, 1973 concert at the Sendesaal in Bremen to commemorate the 50^th anniversary of this concert, released by ECM as Solo Concerts: Bremen / Lausanne.

Here's an (automatic) translation from the article above:

Jarrett Unheard: Archive Discovery

On July 12, 1973, Keith Jarrett performed one of his first piano solo concerts at the Bremen Sendesaal. Despite being plagued by back pain, he impressed the audience with his two sets, which can now be heard on the ECM release "Concerts: Bremen / Lausanne."

However, the second half of the first set has remained unreleased to this day and slumbers in the archive. In this improvised half, Jarrett transforms the romantic tone into an energetic boogie-woogie improvisation.

On the occasion of the 50^th anniversary of this concert, Keith Jarrett's tones, which until now have only been heard by a few, are finally to be presented to the public.

The 40^th anniversary of that concert was already commemorated by a "virtual concert" at the Sendesaal:

Forty years later, on July 12, 2013, a “virtual concert” (that is, without Keith Jarrett) will take place at the same venue. A complete recording of the 1973 concert will be played, “including the unpublished portion of the first set” (“inklusive des unveröffentlichten Teils des ersten Sets”).

The broadcast will take place on Wednesday, July 12, 2023 at 23:30 on BR-Klassik.

Thanks to Matthias for the information!

Update (July 22, 2023). A recording of the broadcast is available online. Thanks to Simon for the link!