I have taken apart 3 original tank valves and have yet to find any packing in them. It seems that when they are all the way open, or all the way closed, they seal metal-to-metal. The stem has a compression style hex nut, which again appears to make a metal-to metal seal when snugged up. Disassembly and thorough cleaning has fixed the leaks for me.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission