Please ignore, I have found what I'm looking for. The answer was to add the following to the section with label
Code:
//echo 'I SEE actionAddProduct: ' . $_POST['products_id'] . '<br>';
Line 1624 in version 1.3.9g of includes/classes/shopping_cart.php

Code:
        if ($new_qty < 20) {
                $new_qty = 20;
        }