BTW, (might be a dumb one) a question regarding this model:
What is the difference between the ones we see with a countdown bezel and the ones with the regular diver's bezel, like this one in the link? If they are the same model number, why the two types of bezel available at their time? Were them the very same model number from Seiko?
6159-7001 is the case number. They share the same case, so it has the same number. This is just like the 7s26-0020. That could refer to an SKX007, SKX009, SKX171, SKX399, SKX401, and the list could go on and on. Some of these are more different than others. Seiko is somewhat inconsistent with this though, for example the 6309-729(x) color variants.