this is base on "search by products model". Now,
you can search products model in the admin search.

in catalog/admin/categories.php

search:
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p.products_id = p2c.products_id and pd.products_name like '%" .
tep_db_input($search) . "%'

Replace with
_________________
p.products_id = p2c.products_id and ( (pd.products_name like '%" . tep_db_input($search) . "%') or (p.products_model like '%" . tep_db_input($search) . "%') )

More...