Since the late 1940's, Mitchell have produced many variations of the classic '300' reel. Spools have developed with each model and whilst each is compatible with others, have subtle differences in design.
Given that Mitchell have produced upwards of 15 million reels, it is easy to just put any spool on any reel and this is what has happened over the last half century. A graphite spool from a 300 PRO just looks wrong on a series 2 half bale from the 1940's.

Looking at these spools, it is easy to identify as to which spool comes from each era,-

For 1/2 bale Pull button ant-reverse.
Series A - Aluminium Bottom, 4 point star drag (deep only)

For 300/301 'butterfly' anti-revs plus 'ottomatics' 350
Series B - Flat Plastic Bottom, 4 point star drag in deep/shallow

For most reels made from 1960 through to 1970.  ie 300, 330 and dark blue 400,410 & match 
Series C - Plastic Star Bottom, small 3 point star drag in deep/shallow/ultra shallow

For most popular reels from 1970 - 1980 including 300a, 300c plus newer 400 series & light blue 410 & Match 440a
Series D - Plastic Star Bottom, larger 3 point star drag in shallow/ultra shallow

For later black models 300, 400, 410 and Match 440a Plus 300 PRO
Series E - Graphite Carbon in Med/shallow

For 810, 840, Anniversary 
Light grey or dark grey

Thats only a generalisation, lots of reels passed quality control with older spools

A few plastic pots, some may have small cracks

Apologies for photo's, If you are confused, ask me a question.
Willing to combine postage, just add to basket and receive extra discount for multi-purchases

Loads of other vintage Mitchell (and other) reels to be listed soon