tune at french pitch

It means that the plate was calculated for A=435, as for all pianos of that era.

When restoring them, one can re-verify the calculation of the plate to have lower tension on the hitch pins when tuning it to 440.

Obviously, given the same type of steel, the specific weight of the material remains the same; therefore, if the piano has strings longer than normal and thus too tense (such as the old Ibachs which had an A4 length of 44 cm), one could lighten the tension by recalculating the thickness, but the string will always remain at the breaking point.

so it seems to me that it is better to tune it to 435. Up until now I have tuned it to 440 but I must say that on the high notes the tuning did not last very long. I thought it was the pins, but from what you are telling me, could it also be that it is actually the structure of the piano that does not allow for greater tension?

No, just as today's pianos hold 444, yours also holds at 440 .... If no strings have broken (especially at the crossing), I would say that the plate is not very tight. Try to stabilize it at 440; on the high notes, the strings are short, so it's normal for them to drop a little more ....

Do the tuning pins feel very loose? I would say that if you don't have the feeling that they turn back too easily and if it has dropped uniformly, it should also hold... is there still some room to tighten the pins a little bit more?

everything holds up well except for some of the treble tuning pins which actually turn too easily and the intonation drops by 10-15 cents in about a week. Regarding the tuning pins, I opened a discussion about a month ago:

http://www.pianoconcerto.it/forum/index.php?/topic/3701-caviglie-troppo-basse/

of course it is that if I tune to 435 those problems decrease significantly

15 cents? That seems normal to me,

anyway, those soft ankles can be fixed.

try the middle ground, 438.... For playing alone at home it's perfectly fine and certain imperfections even disappear