Looks like they used circlip pliers and possibly a cold chisel in some spots. There are certainly witness marks wher they filed the burrs off.
One wonders what they thought they could achieve in there especially since they neither had the skills or the tools to get the back off without damaging it…..perhaps they made some fine adjustments with a 4lb hammer and a cold chisel or maybe something gentle like a 9” angle grinder!
They definitely didn’t clean anything….. it’s putrid!
.