Aftermarket case back. Movement also modified for the fun of it to have a metal brake. I'm really curious what the OP originally said and why it was "deleted."
Photography is not really my thing as we can tell by my many crap pictures previously posted. This is my FOIS where my mate engraved the First Fart in Space with Snoopy letting one rip.
Omega sapphire back on a 3575.20 date/moon Speedmaster from ca. 2002, reconditioned cal. 1866 by Adam Lewis.