You will need to show a picture of the broken spring

Here are some
http://www.dixiegunworks.com/advanced_search_result.php?keywords=top+lever+spring

TS7706 MAYBE??

Catalog says "Breechloading shotguns with outside hammers from 1870 to 1900 had an action that opened with the top lever above the wrist. The spring that actuates this lever is often broken. Has a barely visible pin."