Carradice do a Super C rack pack, if that's the aesthetic you're looking for; the Super C bag Ed posted above looks like a Super C Audax, and looks to be looped onto the bars with some webbing straps and underneath with some straps (sewn on to the bag?) Doesn't look all that stable, but then it's not likely to be all that heavily loaded.
Carradice do a Super C rack pack, if that's the aesthetic you're looking for; the Super C bag Ed posted above looks like a Super C Audax, and looks to be looped onto the bars with some webbing straps and underneath with some straps (sewn on to the bag?) Doesn't look all that stable, but then it's not likely to be all that heavily loaded.