Cannondale six13 52cm Carbon top tube? One for the Cannondale nuts:

I've got a real soft spot for the Cannondale six13 and have been after one for ages. A 52cm has popped up for sale which would be ideal but I've noticed that it has a aluminium top tube rather than the carbon top tube I've seen on other examples.

Is this because of the size? Do all 52cm six13's have aluminium top tubes? Or did they change at some point? It has the carbon down tube like all other six13's.
From my research on your question, it seems that the 52cm Cannondale six13 Road Bikes should have aluminum head tube, seat tube and rear triangle, with the top tube and down tube being carbon fiber. Here's a very good link that describes the Cannondale six13. 

http://www.roadbikereview.com/cat/latest-bikes/road-bike/cannondale/six13/prd_321143_5668crx.aspx