Singer did have their own seatposts, with external expanders. These usually were chopped and modified seatpins, later Simplex posts, to create the wedge that expands the seatpost. The post on the e-bay bike looks distinctively homemade.
I wonder whether the bike is one of 3 (or 4, 5, or 6, don't know exactly) that Rodriquez imported in 1976... I have one, and it is a lovely machine. Not quite the French randonneuring bike, but a great ride unequalled by most others. Furthermore, the workmanship on mine is wonderful. I equipped it with fenders (enough clearance there is) and a TA front rack, which makes for a useful long-distance machine - the racing geometry of the time was very much more stable and pleasant than the later Italian and American "criterium" bikes. Of course, I'd prefer the fully equipped bike with lights, racks, etc., but those are hard to find. Anybody have one for sale in 59 or 60?
I talked about this at length with M. Csuka, the "constructeur," and he agrees that for a useful bike, the 1970s ones are hard to beat. The geometry was totally dialed-in by then, the classic parts all existed (Mafac brakes, Jubilee derailleurs or what have you), while some parts that later were no longer around still existed, notably the super-skinny and thus comfortable fork blades. Earlier bikes are terrific and cool, but difficult to keep up without the right parts, while later ones lack some of the great ingredients. All of them are great, though.
Jan