Don't know to be honest - there doesn't seem to be any consensus but it appears neck profiles changed as have the hardware components - I think the earlier models with the crackle finished anodised parts look the best personally... there's a thread on TB about the changes during which years but its's not very clear. I think the main differences are things like no longer tilt neck, plastic pick guard, different headstock, rosewood fingerboards... not an expert but we all know Leo liked to change things now and then. There certainly doesn't;t seem to be a 'pre CBS' equivalent cut off point or anything like that, though.