I just found the following in the VFP Help file under "table file structure":

Remarks
Visual FoxPro modifies the table header when you turn on or add autoincrementing for field values.

Visual FoxPro does not modify the header of a file that has been saved to a FoxPro 2.x file format unless one of the following features has been added to the file:

Null value support
DateTime, Currency, and Double data types
CHAR or MEMO field is marked as Binary
A table is added to a database (.dbc) file
Tip You can use the following formula to return the number of fields in a table file: (x – 296/32). In the formula, x is the position of the first record (bytes 8 to 9 in the table header record), 296 is 263 (backlink info) + 1 (header record terminator) + 32 (first field subrecord), and 32 is the length of a field subrecord.