find a Professional Bike Shop, with an adjustable reamer, a machinist's tool.
that is what the picture shows.. it is not a Campag tool.

with your seat post in hand, the shop can shave out
just the precise thickness, required, till the post fits..

I have done this , myself, to modify my Bridgestone RB1, frame
from 27.0, to fit the 27.2 post I wished to use.

I kept trying to get it to fit , a cut of maybe .01mm at a time, till it fit,
then stopped.
that way the cutting result was just right..