The faucets are apparently Italian. Very intricate O-ring valve assembly. The brand appears to be something like Elli Roma.
There is a main shaft with three holes: hot cold, and spout.
The water is mixed and flows to the spout hole and from there into a channel around the shaft at the height of the spout inlet. There is a corrugated O-ring above and below the channel to prevent leakage.
Today it started leaking.
We could not find an exact replacement.
So we took two regular O-rings:
• 30 mm i.d., 3 mm thickness
• 30 mm i.d., 2.5 mm thickness
and put them in the channel for one of the corrugated O-rings and repeated for the other.