Trying to alter SKU field on ubercart product that is also a course
The ID for a node form is built up in node_forms():

$forms[$type . '_node_form']['callback'] = 'node_form';


Where $type is the name of each type retrieved from node_type_get_types().

So generally speaking, if the content type is product, the form ID will be product_node_form, if the type is course the form ID will be course_node_form, and so on.