MUSS is the Manchester University Software System. It is an operating system that was designed and built at Manchester University for MU5. It was designed to be portable, and was ported to MU6-G, PDP-11, VAX and MC68000. Some computer manufacturers of the time also took an interest.
A tape of source code was recovered in April 2018, there is more about the story here. The code cannot be published yet (waiting for permission from Manchester University).
The long term aim of this project is to get MUSS running once more. Since the recovered source code is from the post-MU5 era, the most likely target is the PDP-11 or VAX.
The initial effort is to build an implementation of FLIP to translate the Flocoder files into compilable MUSL source code, and a MUSL to C compiler that can be used to compile MUSL generated from Flocoder to C. Then the MUSL to C compiler can be used to compile the original FLIP MUSL code to get an original FLIP into C code that runs.