Since I already responded to your post in another section, I will repeat what I said before: The little piece you lost is a custom made piece and you can't buy it at Brownells or anyplace else.
It has to be fitted and timed so the release hook will hold it and disengage at the right instant. If not done right, the gun can "shoot through," that is
the hook doesn't make contact and the gun fires when you set the trigger. If the contact is too hard, the hook will drag, and the release will be too slow.
Unless can find the lost part, you will have to send the trigger to a gunsmith who
does 1100 release triggers. It's not a DIY project.