Measured the frame...
Tup tube: 556 mm
Head tube: 158 mm (between headset cups)
Seat tube: 592 mm (center of BB to top - seat clamp included); 580 mm (center of BB to underside of the seat clamp)
What is this?! A 58 frame with 57 head and top tube?!?
Ok, if it really is a 556mm top tube with a 158mm headtube, then it sounds like you've got essentially a 57 frame with a 58 seat tube. Not sure how this might have come about. I've seen for example, someone wanting a certain size frame with a shorter headtube to get lower perhaps, but still retaining the top tube length, but I've never heard of anyone (or understand why they would) choose a certain size frame with the only modification being to extned the seat tube by 1cm. Perhaps you'll never know the real answer to this question. But if you're happy with how it feels, then it's all good. God knows how much we all know how you'd like more seatpost showing, and I think it's become kind of entertaining actually. Your saddle to handlebar drop of 8.5cm (I think you said that's what is is) is a very reasonable number. I know I couldn't ride with any more drop than that using classic bars and still have good use of the drops.
Take one more measurement, just for kicks. Measure the wheelbase, by measuring from the center of the skewer above the rear derailleur to the center of the skewer on the front wheel, keeping the front wheel in perfect alignment with the frame. It's generally pretty easy to get a straight line right across those two points, since the ends of the skewers are small enough to get good center points. What is it?
I love this thread for the pics of colnagos... so sorry in advance for getting way off topic here, so PM me if you like.