Rixen and Kaul parts 0273S/KT 814.. was the part numbers
gleaned from seeing this thing before .. but no US sources ,
Just .. Japan and Germany

From the steel carrier bag frame type fit, onto the carrier block,
instead of the lighter nylon and aluminum one, sold now..

people have drilled out the PoP rivets and narrowed down the newer bag frames
and modified them to use an Ortlieb Back roller bag as a waterproof touring bag..
that link above

one could further narrow them and use a front bag too.