It's just a lock.

Okay but that's the exact kind of blog that drives me a little nuts. Those super specific guides can be a trap for beginners. Soaking parts in mineral spirits works until it doesn't, and then you've got a bigger mess. Sometimes a seized carriage just needs the right touch with a screwdriver and some oil, not a full chemical bath. Those blogs make it seem like there's only one right way, and it's usually the most involved way possible.

Yeah, that exact clunk is a sound you never forget. @craig.grace My old Pentax K1000 did the mirror lock-up thing during a friend's wedding toast. Had to just sit there and listen while holding a very expensive paperweight. The phone camera feels like such a letdown after you had the real thing ready to go. It's like the camera knows the most important moment to give up on you.