I had to solve a very similar problem extracting data from a Microfoucs Cobol .idx file. This was a Y2K project in which we rewrote a COBOL system in VFP.

These hints may help.
You will need a hexadecimal editor to examine the contents of the file.

1. Regarding numeric fields:

Each byte of the field represents two decimal digits of the mantissa of the number. The rightmost hexadecimal digit represents the sign. e.g. a value in HEXADECIMAL notation looks like:

00 17 50 0C
This represents the DECIMAL number 17500

The last digit in the hexadecimal number determines the sign
C for positive, D for negative.

The interpretation of where the decimal point goes depends on how the field was declared.

The above field was declared with
C-FEE PIC S9(5)V99 COMP-3.

and thus it has 2 decimal places, so the above hex number represents
+175.00

2. What if you don't have the FD's ?(I didn't either). I did have the compiled COBOL program which provided functions to view data. I could for example look at a transaction with the COBOL program and note that the fee was $175.00. Then I found the record and field in the actual file using a hex editor, noted the offset from the beginning of the record. In this way I was able to deduce the structure of the .idx file. This is admitedly laborious and a last resort, but it works if you can't locate the COBOL source and can't decompile the executable program.