Worth checking
this page. The "Prontor-S" refers to the shutter mechanism but this page shows Prontor-S only on 75mm lenses..... further down, a "Pronto" shutter is listed with a 105mm lens. One of those two options, I suspect. Depends on whether the camera take 6x6 (cm) or 6x9. There's a Forum on that site..... worth asking there - but the combination of lens and shutter on your model doesn't quite seem to fit in.