Vague spec gleaned from somewhere (Kermit I think).

ANSI X3.27 - 1978
Magnetic Tape Labels and File Structure for Information Exchange

specifies format to be used is 1600 bpi, 9 track, odd parity,
and ASCII coding (as per ANSI X3.4 - 1968).

80-byte special records

Volume identifier (tape label) begins with "VOL1" in columns 1-4,
followed by six-character alpnumeric label in columns 5-10.  Remaining
columns not defined.

Each file prefixed by a record beginning with "HDR1", and containing
the file name in columns 5-21 (17 characters, presumably space-filled
and left-aligned).  This is followed by a second record beginning
with "HDR2", a format identifier in column 5 (either F, D, or S),
followed by two decimal numeric fields (zero-filled, right-aligned):
block length in cols 6-10, max record length in cols 11-15.

Format D presents data in variable length records, each beginning with
a 4-byte length field (decimal, zero-filled, right-aligned) giving the
actual length of each record (including the 4-byte prefix).  The format
is intended for text files, the record separators (eg NL) are not included
in the data.  Records are not split across tape blocks, so last records of
each block may be padded out with '^' characters (not included in the count).

The two HDR records are followed by a tape mark, the file blocks, another
tape mark, optionally an EOF1 and an EOF2 block.  The next file follows
without an extra TM.

The last file may be followed by EOV1 and EOV2 blocks, which is finally
followed by a double tape mark.
