Normal wear marks... Mine has them, as does most every other 500 I've looked at. My bet is you just haven't noticed them before.
What I'm not sure of is whether they are from the bolt's action or from shell rims during ejection? That is all I'm curious about.
Dude, your fretting over a non-issue.
Steel bolt is going to scrape/gouge an aluminum receiver, it is going to do this anywhere it contacts.
Most will get small nicks, dings, gouges and scratches/scrapes here and there inside the receiver.
As long as it is just minor scrapes/gouges, it is OK, I wouldn't worry about anything but cracks or penetrating scrapes/gouges.
Each one is different, consider it a character blemish distinguishing it as yours.
Here look at mine, almost the same as yours, except opposite ejection port yours is scraped and mine scratched.
Shell feed, bolt locks, pull trigger and gun goes BOOM, cycle the action, shell ejects... Repeat... Gun works fine.