OH boy, are the opinions going to fly! LOL!
For the checker bound 4001's, the 63-66 period are my favorites.
Aside from the long headstock, you get the square heal and smooth carve at the nut area.
There are many distinct yet subtle changes within this period such as horseshoe pickup bobbins, mounting plates, and G-Tuner placement.
These are skinnier necks, they're not the skinniest (I've played a '62 that was scary thin..), but the majority have a nice shallow D profile (not all though...).
This period also tends to be the most pricey due to their general rarity over the later ones.
67-8 has a thick bump extending from the headstock to just past the nut. This adds strength, but is not as graceful. Also, these got the round heal.
The treble pickup surround changed to the rectangular shape as well. The neck profiles tend to be more C shaped, and to me feel a little thicker(though there are some thin necks to be found as well....)
These are less rare and (with an exception or two...) tend to be a little less $$$$
As with all early Ricks, no two are alike, so it would be hard to make further generalizations.
Fireglo's very from Orangy to deeper reds. The 63s I've seen and the 67s were on the lighter side, but I am sure there are many dark ones from those years...
Price being an issue, the 72-73s are the most affordable and abundant. You loose features like the long headstock, one piece neck, and horseshoe pickup(well this was officially changed in 68 though a very few were installed on later basses...), but you gain cool things like Grover Contour Slimline Tuners, and the Shedua "skunk stripe" neck has a nice tone and look. The necks are a little thicker, but very nice.
So, that's one opinion. You'll likely get more....
