This is the v2.1 of CP/M for DEC Rainbow.

First: uncompress the 'cpmv21.lzh' file with 'LHA'.

Then run the Teledisk program on a PC (IBM type) with a 5.25" 1.2Mbyte drive.
Insert an RX50 floppy and use the 'cpmv21.td0' file as input for the
file to disk option.

'Teledisk' and 'LHA' can be found in the ibmpc/util directory.