The following is what I think it should have been, but even after trying ALL of the "letters remaining" for the one letter left over, nothing worked! So I just gave up and never found out what it should have said!
If you know, please post!
"Safety _eans first aid to the uninsured." - Unattributed
The _ is for the missing letter.
No, "m" did not work
I believe the "letters remaining" were: b c g h j k l m p q v w x y z
Also, all of the letters in the quote are repeated at least once except the missing letter and the "h" in the, and the "o" in to.