We see that. Some seem to do it more than others. Back in the day the striker channel sometimes came with leftover machining debri but haven't heard of that.
That looks like brass probably from the striker smear. Many flush out the striker channel with brake cleaner. This might prevent the build up that your seeing.
I generally don't myself.
I just take it down annually or so and so far I've not had any issue.
In Memory of Paul "Dietrich" Stines.
Dad: Say something nice to your cousin Shirley
Dietrich: For a fat girl you sure don't sweat much.
Cue sound of Head slap.
RIP Muggsy & TMan
"If you are a warrior legally authorized to carry a weapon and you step outside without that weapon, then you become a sheep, pretending that JOCKO will not come today."