Colnago Geometry

Matthewn5 has pretty much nailed it

The H models (longer headtube) were first released on the C60 and allows you to run your stem slammed  but still maintain the same stack height of a normal C60/64 that has around 20mm of spacers fitted

Sloping geometry....on a Colnago....I've just spat my tea over my laptop! 

Back in the day Colnago's bikes were only available with a traditional (horizontal) top tube. Then at some point they were offered in Sloping (modern) or traditional (horizontal) geometry. The sloping geometry was designated by an 'S' after the number. The number refers to the seat tube centre-top dimension, so a 52s ('sloping') has a 52cm seat tube but the same effective top tube (550mm) as a 56 ('traditional'). Those number designations cause a lot of confusion to this day.

Here for example is the geometry chart for the C59 with the 'sloping' geometry on the top chart and 'traditional' below:

The C60 was the last to include the traditional (horizontal top tube) option:

The C64 finally dropped the traditional option, retaining only 'sloping' and 'high' geometries. The 'high' geometry also features a sloping top tube. Here's the geometry chart for the C64 with 'high' and 'sloping' geometry shown:

The geometries are indicated in the sizing by 52s or 52h. Both 52h and 52s have the same seat tube length (52cm), but the stack is higher for the high version and the reach slightly less. This is achieved by putting more slope on the top tube, and fitting a longer head tube.

They have an explanation on their website as to why they now only produce bikes with sloping top tubes:

https://www.colnago.com/en/news-en/road-news-en/the-reason-for-sloping/

I'm sure someone will be along soon to correct this, but that's as I understand it.

If you're interested in the evolution of Colnago's carbon bikes, this is packed with info (pdf):

http://brown-snout.com/cycling/tech/_colnago-c40-family-tree.pdf