Seat tubes have a standard outside diameter of 28.6mm. 29mm is close enough to indicate it is not one of the exceptions. The resulting seat post diameter is determined by the thickness of the tubing, which is also an indicator of the level of the tubing. Larger seat post diameters are indicative of stronger, higher level tubing with a thinner wall. If it is Tange tubing like the North American models, it should use a 26.8mm post. If it uses Europen tubing, the post may be as large as 27.2mm. Taking it to an LBS for measurement, is the safest approach.