Juan,
AFAIK, Rodenstock Sironars were never inteded to be useed as a convertible (except maybe very old ones from the 60's). I assume that the previous owner had changed the f-stop-scale. Plain Sironars usually have the same coverage as the current APO-Sironar-N-Series. The APO-Sironar-N 5,6/210 has got an image circle of 301mm. Rodenstock is usually conservative in stating technical specifications.