FLX is a free utility written by Paul Koning (former RSTS developer at DEC,
now at EqualLogic) which can read/write RSTS disk images, better than PUTR
can.  It hasn't been updated in a while, but neither has RSTS.  It comes
with C sources.  The .zip file contains the manual as a word document and
as a postscript file, and flx.pdf is an Adobe Acrobat version of it.