'If the comment that 'any 201 can be retrofitted with
a B stamp' is really true...'

I think that is implying that any 201 can have the
ballast system added, and the flight manual revised
to show the higher gross weight permitted with water
ballast. My machine is having troubles openning .pdfs
right now, so I cannot tell you which Tech Note this
is. It is not a 'true' conversion to the B model,
with changed dive brakes, tail, etc. Although, there
is a Tech Note that specifically, by serial number,
installed the newer tail on an older serial number
201.

Steve