These are images with which you can IPL Hercules, the S/370 emulator,
and include DASD images, etc.
From the Hercules FAQ:
3. Alternatively, you can download the mftr33.tar.gz package which
contains a minimal MFT system on a 3330 image, with some minimal
documentation. The configuration is that of a 370/145 with 2
megabytes of main storage, running OS/MFT Release 21.0, with several
volumes of pre-loaded DASD. 2 program partitions (P0 and P1) have
been genned. There is no working TOD clock, and no SMF. Volume
DLIB01 which is specified does not exist in the zipped file, but can
be created using a plf file with the CDROM of OS/360. DLIB01 can be
commented-out from the config (cnf) file if desired. The mftr33
package also includes the IPL text and program for the standalone
IBCDASD program as file "ibcdasdi.rdr" which can be used if desired.
