Hmmm, according to Mars they both have a FET preamp and a transformerless output. My interest is primarily in the 63, in the hope that it's more like the MXL-V67M rather than the 57/1006 and the 2001. Jeez Marshall, can we get some more model numbers somehow?
The 67 has a transformer-balanced output, which may be a crucial component of its sound. Somewhere I've read that the actual shape of the 67's microphone body/grille had an impact on its sound as well, so the 63, with the same body, should at least share those characteristics.