Slightly different mount size.  KL lenses are made to fit the new RB (Pro SD)
with a wider mount hole to allow for mounting up the 500mm as well as the
tilt-shift lens.  With the adapter in place that comes with the lens, the KLs
will mount to the RB and the RB Pro-S.

Not sure if there are any other differences internally...take a look at the MAC
web site for more detailed info.

--
Scott Wuerch
Mind's Eye Photography
wuerchs@worldnet.att.net

Computer problems..have you checked the loose nut directly in front of the
keyboard?