Just when you think your gonna get past all this weather, crap like this happens! 1 /1/2 " incoming water check valve. This came in from the city water line, and shut down my water, turning off my weep system in 15* weather! now everything is frozen!
That looks like part of the valve itself. There is probably two of those valves in your backflow preventer. If that one is intact, that debris is probably part of the other one.
The last time my backflow preventer was a part I remember a piece exactly like that being in there so that's correct it's part of your backflow preventer.