The bulb holding tailcap was an option on second gen Kel-Lites, but may have been installed on a new old stock first gen.
The one in question is a late model big head 1st generation light. And the tailcap may have been made by Tony Maglica who made some of the tailcaps whenever Kel-Lite needed him to make some due to more demand than Kel-Lite could handle. First gen were made up to 1974 even though the second gen began in '73.
The tailcap with the Kel-Lite logo is pretty rare. Good score!
Tony Maglica had a machine shop for making brass shells for the US Navy back then. He and Don Keller colaborated on a number of things including prior to Maglite. Don worked for Tony twice. They conceived the first krypton flashlight bulbs.