There are two that fit your description but Google 1187 diagram and I got this link

http://www.mcclellandgun.com/Gunsmit...1187Parts.html

If it is #56 (link) that slides and fits behind the bolt and the end pushes down on the return spring mechanism which is in the stock or it might be part # 39 which is the feed latch and that snaps into the side of the receiver. This is usually peened in but can pop out do to a poor side stamp.

Or as TG said take it to a gunsmith.

JW