Would this bike have a 27.0 or a 27.2 post. It appears the 27.2 post can be installed but feels a bit tight to me...Before I do any serious damage I'd like to know. I think it is a p13 model I am not sure. I see on the Sheldon Seatpost database that the 72 Paramount track is 27.2

I am hoping that it is going to be okay.

It's 27.2.

The seatpost end of the 28.6 O.D. 531 seat tube has a 0.6 mm wall thickness. 2x0.6mm = 1.2mm, which, when subtracted from 28.6mm, yields an I.D. of 27.4mm. However, the tolerance on the wall thickness and roundness of the seat tube made it so that you rarely could fit a 27.4mm post inside, even before brazing. And then, the seat tube always got distorted during brazing, making it even less possible to fit a 27.4mm in there, but a 27.2mm fit nicely.

The same goes for why a 27.0mm seat-post was often used on the bigger sizes, which were made out of Columbus SP or SPX, whose seat tubes had 1.0 X 0.7mm wall thicknesses. Because of tolerances, the predicted 27.2mm post (28.6 – 2x0.7 = 27.2mm) never fit, but a 27.0mm fit nicely.

The 1972 P13-9 Specifications Sheet shows the Part Number for the seat post is 58 910, and there was no change between the 1972 and 1973 P13-9 seat posts. The Parts and Accessories Catalog describes P/N 58 910 as "Campagnolo seat post, complete, 27.2 mm"

Hi,
.
My 1972 Paramount P-13 has a 27.2 seat post. It's a snug
fit but adjusts easily without scarring. It's heavily
greased, as it should be.
.

27.2 on my '70, 27.0 on my '61.

-Kurt