It's definitely metal shaving away. I took a few pictures and couldn't get the lighting right, so it's hard to tell, but it's very clearly a series of scratches in the forward/backward direction. Here's a zoomed picture of the extractor.
I spent 10 minutes trying to figure out where it rubs, and couldn't tell for sure. There's definitely wiggle room in the gun between the top and bottom when assembled - it rattles slightly when shaken vigorously side to side. At the wrong angle the extractor hits something when sliding into the top during assembly - I'm guessing it's the ridge along the bottom face of the slide...?
In any case, I'm glad it's not normal - hopefully this explains the recent stovepipes. I've got a spare housing with 336 ejector that I
was going to use to replace the chipped ejector on my jam-o-matic 19Gen2. But I'd rather regain confidence in my main gun, so the jam-o-matic will remain my remedial action practice gun for now, and I'll play Dr. House with that gun later.
On a related note - does anyone know where I can order a 9mm gen4 trigger housing with extractor? All the usual places I know of are sold out (Glockmeister doesn't say so, but IME they list out-of-stock parts without telling you they're out of stock...)
Thanks!