Vidas and I are preparing to perform in Sweden this summer. We will be playing an instrument from Sweelinck's time.
It stands in Stockholm's German St Gertrude church (the Duben organ) and has all kinds of features of old organs: mean-tone temperament, split keys, high-pitched tuning, and short octave.
A short octave in this case are the 3 diatonical keys in the bass octave - C, D, and E. This means music with C# and D# in this octave cannot be played.
So we chose our repertoire very carefully, avoiding pieces with more than 1 accidental.
But it's still a challenge to master the new layout of this type of keyboard. In the above picture you can see how a similar keyboard with CDE short octave looks like.
It works this way:
The lowest note which looks like E is actually C.
D looks like F#.
E looks like G#.
F looks like F. Great!
F# is the additional semitone on top of the 1st sharp.
G is G. Great!
G# is the additional semitone on top of the 2nd sharp.
From A everything looks normal again.
You might already feel that adjusting to this short octave will take some time and will require some special fingering.
It takes more than that. We will be circling with pencil all our notes in the bass octave of our scores which would require re-positioning. And then we will be practicing on the modern organ or piano the way it would work on the old organ in Stockholm.
The result will not be pleasant (when we need C, we'll play E; when we need D, we'll play F# etc.).
But this is the only way to get used to the short octave on the target organ and shorten the time needed to adjust.