The shirt is one of those from tobi.com. You can see more details about it under Postscript to PS Shirt.
The shirt has darts on the back, but in my experience the shirt made for tobi.com were a more generous real-world fit.
Paul Smith’s reissuing of the Burgundy shirt in October 2010 increased their number floating around, but the blue ones are becoming increasingly hard to find.
You can email your interest with an offer to firstname.lastname@example.org