Well I am astounded at that late date of production, they have no data at the manufacturer. Maybe you could look in something like an early 60's Gun Digest or something that has a gun catalog in the back and see if they list any Heym offerings. That is, if they were being imported at that time. If not, then you are gonna have to learn to speak German and post on some European gun websites, I suppose.
Your gun is like your nose, it is just wrong for someone else to pick it for you!