Do a search for Randonneur or Audax bicycles. When you get to the Rene Herse bikes, stop looking.
A 50-28 double? I've not seen such a beast. I have a 50-34 compact(110 bcd)
Velo-Orange sells a 46-30 in the C & V approved 50.4 bcd
http://store.velo-orange.com/index.p...-crankset.html
I discovered that if you buy the VO 50.4 crankarms, you can then buy a 50T outer and a 28T inner ring. wow.