The expert's opinion these days tell us that there are 9 versions of the 300 that were manufactured in France. An excellent site to find detailed information on each version is at http://www.mitchellreelmuseum.com/. The website also includes an excellent forum where you can ask questions of people who know the history of these reels, and the company the made them.
Due to reels being sent in for repairs, the factory using up old parts, overlapping of production, efforts to build up rarer versions by unscrupulous individuals, and trips in for service, it is very possible to find parts from more than one version on your reel.
A serial number on the housing will allow you to get close to what version, and date of manufacture at least that housing part is. These numbers, or a letter and numbers can be found on the side of the housing, or on top of the foot. It is also possible to find just a letter on the foot as well. Some detective work on your part may add to your knowledge about the reel.
From left to right, versions 3, 4, 5.
1st version Circa 1939
2nd version 1940
3rd version 1946
4th version 1950
5th version 1955
6th version 1958
7th version 1968
8th version 1977
end of production
in France 1989
Link to Mitchell Museum is down!