Hans van Oost asks how one should describe the device that holds down
the music over the tracker bar or keys in a keyframe.

In a true keyframe, that is, one that has keys, I would use the
description 'grooved roller'.  I would not call it a pressure roller
as it should be positioned to hover above the keys and _not_ push the
card down against the bed of the keyframe, although it does, of course
resist the upwards force of the keys acting on the card.  When the card
goes through, there should be a clear gap beneath the card.

In a keyless frame, where a small downwards force is required to seal
the card against the tracker bar holes, if I were to use the term
'pressure roller' it could be confused with the rubber covered roller,
or pinch roller, that drives the card through.  Therefore I would also
use the term 'grooved roller'.

The pedants may point out that some small Limonaire organs use a closely
spaced pair of plain rollers in place of the grooved roller but that
shouldn't stop us using this common terminology.  So my vote goes to
'grooved roller'.

Best wishes from Great Britain,
Nicholas Simons