They are practically identical. DS1500 was designed before we have DS1510 and it uses a different basket design (slighly larger). After DS1510 was designed, we decide to use the same basket for all 15" models so they are swappable. So we edesigned DS1500 into DS1501. We also re-tool the surround so that it looks better. But T/S parameters and voice coil are the same.
Thanks, because I just bought a used F15 and then I realized it has the older driver.