The RE50-b, 635a, 635a-b all share the same element, which is tuned exactly the same for each model.
The final response curves are not quite the same between the RE50-b and either 635a but very very close.
The RE50-b and RE50n/d-b are completely different mics with the exception to the coil, diaphragm, outer shell, XLR insert and blast filter. The magnetic structure are entirely different.
In my opinion they sound like completely different mics, with the RE50b being preferred. The sonic differences are great and obvious.
Often I find the n/dym's magnetic force is so strong, it will literally pull the magnet into the gap, hard, ruining the coil. This happens from a drop or fall. Don't think I've ever seen a failure of this type in the older alnico magnetic circuits.
FWIW I own quite a few 635a, and RE50 mics, none of them are the n/dym models.