[cctalk] DEC RSX BACKUP.SYS (bup?) files

I've got a few RX50 floppies that are *part* of a couple of backup sets. I don't have the complete sets, so the backup utility (BUP?) won't process the set in bits and pieces. Is the BACKUP.SYS file format documented anywhere? This is the file that starts out "JOURNAL", a few nulls, then "MICRORSX" Any help would be appreciated! TIA, Chuck