Good morning, why do you think this piano (German, late '800, bayonet action) has 88 keys and 89 notes? As you can see from the photos, it has C#8, only the three strings but not the corresponding key (obviously).
Hi. Let's try to reason through it; perhaps I would tend to exclude that both the cast iron plate and the soundboard were changed, as the latter actually exceeds by 3 tuning pins.... I can make two hypotheses: either the keyboard block was changed along with the action block, replacing it with one of 88 keys instead of the original 89, perhaps taken from a piano with a depleted or cracked soundboard. Or perhaps the manufacturer deliberately left three strings so that they resonate by sympathy and there is a bit more ensemble effect. This is what I think.
I'll try to give my opinion. I have noticed that the first and last notes are always somewhat unintelligible, even compared to the second and the penultimate ones. Maybe this could be the reason.