Most manufacturers determine frame size based on the seat tube. For a model with a traditional, horizontal top tube there are two methods. First, is the length measured along the centerline of the seat tube, from the center of the BB shell to the intersection with the centerline of the top ube. This is commonly refered to as center-to-center (ctc). The other method is the same except that it measures to the top of the seat tube and is commonly refered to as center-to-top (ctt). I beleive that Centurion used the latter method. During this period Centurion offered 46, 53, 56, 59 and 63cm frames. Based on the top tube measurement, it sounds like you have a 56cm.