These notes are from the last item (9999/001) in the D4.2B driver file:
 
          MTS RD4.2B   tape 1 of 1   9-track   1600 bpi   *FS tape
           VOLUME=4.2BT1   LBLTYPE=VLO   REVL=E          

           The UM driver file used to create this *FS tape was the last
           file saved on the tape.  To make additional copies of this
           listing use *FS to restore the driver file, file number (88);
           and use the *DEDIT command LIST ON *PRINT*.     

           This tape includes:

             (1)  the *CDUPDATE decks for MTS and friends (MTSUPD, CMDSUPD,
                  RSFUPD, ...),

             (2)  several components with changes that are required because
                  of the MTS changes (HASP, *SMDS, GLOBALS, SIGNONM,
                  and STARTUP/SHUTDOWN),

             (3)  the *CDUPDATE decks for UMMPS and CONFIG,

             (4)  several components with changes that are required because
                  of the UMMPS and CONFIG changes (TABLES, ONLINE/OFFLINE,
                  LLMPSEQU, FAKEOS SVC 59 simulation,  Machine
                  Check Recovery, EREP, and UNITS),

             (5)  PDLIST because I got confused and thought the changes
                  George made were required by (4) when in fact they weren't,

             (6)  other components that are needed to assemble the above
                  (*ASMH, *SYSMAC, COPY:MISC.MACROS, COPY:CC.MACROS and
                  COPY:SETPARM), 

             (7)  a preview version of the incomplete disk manager,
             (8)  print files for recent items from MTS:FORUM and TXTF:FORUM.   

           The idea (or at least the original idea) behind this redistribution
           was to send machine readable *CDUPDATE decks with MTS and UMMPS
           changes rather than paper change forms.  

           This is the first redistribution of MTS to use the new numbering
           scheme.  Rather than RD17 the name becomes RD4.2B. RD16 would have
           been RD4.2A under the new scheme.  Because tape volume names are
           limited to a maximum of six characters and an additional two
           characters (Tn) must be appended to the name to specify the
           tape number it has become necessary to drop the prefix RD from
           the volume name (VOL=4.2BT1).  Using this scheme the volume names
           for D4.2 of MTS would have been VOL=4.2Tn. 

           RD4.2C is tentatively planned for late 1979.  Tapes from
           other installations to be included on RD4.2C should be sent to
           Liz Sweet and arrive in Ann Arbor by 26 November 1979. 

           Jeff Ogden (UM)