This happens as the die is abraded, lapped, polished to remove either a clash or just clean up the surfaces to reuse. The low relief details are the first to be removed. The FG is still visible at least some of it.
I can see a small piece of the FG but it also appears to have die cracks on both sides of the floating roof.
Late 1960's and early 1970's You can find various Floating Roof issue. It's not recognized by the grading companies and there is no premium for that issue. It is the result of an overpolished die. It is not a variety, it is not a mint error. It is just a late die stage of that particular die, but could also happen on other dies if they are overpolished.
I have so many of the 1970 s and the 1969 s that I gave some to friends that think they're cool looking but people say there is no value to them but face value
