Probably the biggest differences are in the head of the bolt construction and the method of extraction. On a pre-64 the extractor is like the Mausers which control the cartridge all the time from pushing the cartridge up out of the box magazine into the chamber and then pulling it out after firing during the extraction/ejection cycle. The post 64 just pushes the case and then as you close the bolt a small bolt face extractor snaps over the case rim. A much weaker system and cheaper to manufacture. This basic change goes further as the bolt manufacturing is easier also so follows cheaper. The post 64 work fine but not the higher quality of the pre-64's.
Hope that helps. Buy pre-64's IMO, Its all I own for bolt guns and I have a few ranging from 30-06 thru the mag cartridges to 375 H&H. One added comment, if you are going to hunt DG the post-64's or the Remington 700, which uses the same extraction as the post-64 are NOT your best choice.
