All Finnish Stugs have the 531 prefix, it was the 'code' for the type in Finnish service.....531-3 was a MIAG manufactured StuG-40 from the first batch of thirty purchased by the Finns, it was destroyed in Nurmilampi, Tali, June 28th 1944:
http://www.andreaslarka.net/sturmi.html
For all your Sturmi needs.