Ancient units of measurement persisted long after 19th century
metrication in Europe, especially in craft industries -- and indeed
still persist. Countries invariably had units corresponding to the
inch, one twelfth (Latin: unica) of a foot, or the width of a thumb.

My 1910 Rönisch piano was clearly designed using the then Leipzig Zoll,
which was (is) slightly larger than the UK and US inches; and my 1924
Steinway the close but different Hamburg Zoll, which explains the small
discrepancy with New York instruments' dimensions.

The Germans use(d) Zoll Wassersäule (inches water column), the French
Pouces de l'eau and the Italians Pollici d'acqua: inches of water.
Nowadays centimetres of water are more usual.

Patrick Handscombe
Wivenhoe, Essex, UK