Christoph, the best advice I can give you is to scour the internet. There is a Winchester Collectors forum that may be helpful as well as assorted shotgun forums.
Please let us know how your search goes. If worse comes to worst, post a picture for me. I may be able to do an invisible repair for you depending on the damage.
__________________ If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen. ― Samuel Adams