The first 'official' Mag-lite(tm) (latter labeled with a (c)and then an (r))was made in 1979. A number of lights were made before this starting in the early 1970's. These lights were not standardized retail products and had various labels that did or did not include Mag-lite, Mag Instrument, Los Angeles and others. There is debate as to what inscription was used first on true production models. What is known is that by around late 1981, the company moved to Ontario, Ca and began to use this location in the label (in place of L. A. or no location). Non 'Ontario' lights are rather rare. Perhaps this info will help with the time frame.
As for sizes, Mag did make both C and D in seven cell. I don't recall the exact date, but by about the mid 1980's the 7D was dropped. The 7C went on into the early 1990's. By special order, up to a 10D and 12C could be had until around 1984. There are rumors of 14C, but I have never seen one (good throw, but outstanding swing!!!). These special order lights came with an adapter (a big 'dummy cell' made of an aluminum rod and two 'washers') to run on four, five or six cells as requested. As Threepio pointed out, the seven cell argon bulbs were about as bright as the five and not desired. (The after market krypton bulbs were brighter but cost a whopping $10 in the early 1980's.)
Today, the six cell is the largest made and, unfortunately, the company has firmly implanted an (*) into M*g /ubbthreads/images/graemlins/smile.gif.
As for head sizes, the 'large' head was the extra-large size and rather rare. The large is the standard (and only) size of today. The small head is the rarest worldwide, while the extra-large is rarest in the USA. Btw, I will pay top $ for a good condition small head (flashlight, that is).
I am not an expert on the history of Mag-lite, but feel free to ask questions. I hope this helps.
Light cannon!
