Okay,

So it seems the seattube does not accept the common 27.2 mikemike seatpost. My metering device is not quite up to scratch, but it seems it could well be either 26.8 or 26.6 (which, according to Sheldon, are common in older Italian frames).

I think I have to haul the frame to a shop to check the diameter unless there is some other way to make sure, i.e. some home remedy you guys know of?