One of the signs accompanying the helmet. It reads
This space helmet was worn by Eugene Cernan during the Apollo 10 mission.
On loan from the National Air and Space Museum, Smithsonian
Institution, Washington, D.C.
Although the sign indicates that it was Cernan's flight helmet (i.e., worn
during the Apollo 10 mission, rather than just during training), the helmet's
headrest is marked with "III", indicating
that it is class III hardware, non-flight hardware relegated to training.
However, budget cuts later in the program often forced NASA to use hardware
flown on earlier missions for training for later missions, so that may be how
this helmet became designated class III.