Here's a shot of the mixte I built for my wife in 1979.
Mostly Reynolds 531, with laterals made from Reynolds 5/8" seat stays spliced to 5/8" plain gauge 4130. Much stiffer in torsion than the typical 12mm laterals of the time.
Braze-ons weren't commonly available in this area in '79. So cable stops were salvaged form Simplex front derailleurs, the pump pin and diamond shaped reinforcement were cut from an old Raleigh 3 speed frame and the brazed on brake pivots were cobbled from bits of seatstay and old rear axles. Cable guides were cut from tubing.
She's a great-grandmother now, and still uses it regularly on club rides up to 50 miles. Over the years, the bike has evolved to meet her current tastes and needs. Tires are fatter, the train wreck shifting has been replaced with a Sturmey Archer S5, saddle is now a Honey Brooks B-66 and pedals are SPD. Due to degenerated lumbar disks, she now needs flat bars with one of those adjustable angle "Comfort" stems sticking straight up. (I probably ought to take a more recent photo.
BTW All the mixtes shown on this thread are sweet, but that Peugeot PE 25 is an absolute stunner.
http://www.flickr.com/photos/35685161@N00/ Shows what may be the nicest mixte ever. Built with 650B wheels, by Peter Wiegle, for his sweetie.