Joakim: The main difference is the hotter output. I live in Lisbon, Portugal.
Gene: Your December 1973 4001 is basically like my April and my June 1974 4001s.
Not to impugn on Seymour's work, but from most reports his Rick-style pickups are less than satisfactory, and from pictures I've seen they deviate a lot from the originals in looks and design. I understand his F****r-style pickups are great, though.
Maybe he should try and be the next Rickcrazy - that way he could come up with the ultimate Rick-style replacement pickups...

A Rickenbacker bass is much like the Jaguar E car - perennially ultra-fashionable.