See here:
https://www.camaros.net/threads/9204...12/post-434954
which says 8519 is a 68 Firebird w/disc brakes. The big font stamps are also pre-69 format I believe.
So I would speculate the shell started life as an 8519 dated 048 (of 68), which did not get shipped out from the Delco Morraine plant for some reason (maybe failed QC test and got laid aside), and then the shell got repurposed into a 9204 dated 110 (of 69). You would think they would have ground off the old stamp somewhere in the process, but apparently not.