If you want to know about your Mitchell 300, check out: http://www.mitchellreels.net/
Great website. You can take the serial number from the reel and prob. find the year it was made. They have a LONG history. I have maybe 5-6 of various years. Yours is a common one I can tell you that because it has a full circle bail, and the metal release on the reel body is of a wing nut shape, which is of the more standard ones.
Not to take away from what you've got though, they are real sweet for their time period. But if you saw one somewhere for $400.00 it was not that model I'm sure.
Edit: Actually, here is a cool link that you can use to date your reel: http://www.mitchellreelcollectors.com/Dating_Reels.html
There is so much online info. on these reels. People are freaky about collecting them. It is the reel that I first fished on, and was handed down from my dad, etc.