Going slowly insane, or keyed file continued

If a physical file has been defined as being comprised of 3 fields:

a character field of length 1
a character field of length 10 that designates the key
and a character field of length 117 that designates the rest of the data

Is it possible for me to generate a different record format for this file and
actually have it work as a keyed file?

The issue is that the 10 character key is actually TWO fields - an 8 character
field and a 2 byte numeric.  I do even care WHAT is stored in the 2 character
numeric field.  I just need to ensure that there is not another record matching
the first 8 characters.

What I need to do is define the record format to look like

a character field of length 1
a character field of length 8 - designated as key
a numeric field 2 bytes long - don't care if this is designated key or not
various length character, numeric, and packed decimal fields to comprise the
remaining 117 bytes

I can make this work with a sequential file and read the data.  However, I can
NOT get my program to recognize the key unless I keep it at 10 characters long,
which is impossible.

Amy