Hi Mehron,
My best guess is that it's a part called a "Regist actuator rear". It appears on page 250 of the ''Brother HL-5240 Service Manual''. Here's a picture from that manual.
[image|2788769]
I ran across it on a site called manualslib.com. You can view it there on the site or download a copy to keep.
[link|https://www.manualslib.com/manual/684564/Brother-Hl-5240.html|BROTHER HL-5240 SERVICE MANUAL Pdf Download|new_window=true]
Since it appears in a section titled "Disassembly Procedure", you should be able to figure out how to put it back in with the help of the manual.
Good luck and let us know how it goes!