If I recall correctly, there are about 17 LANGUAGE php files where "model" is defined.
This one is out of advanced_search.php
Once you have found the relevant files with these DEFINE STATEMENTS, you will edit them like this:
After editing, the files should not be allowed to overwrite the CORE files, but should be ftp'd to your respective CUSTOM folders (over-rides folders).
define('TABLE_HEADING_MODEL', 'Part Number');
If you don't know what I mean by "over-rides", then please look in the FAQ for CUSTOM TEMPLATE SETUP before you dive headlong into carving up files.