Custom Rewrite Rule for Custom Post Type with URL Param

SOLUTION:

I am answering my own question here since no responses were given and I presume this might be helpful to someone and in the mean while I found a solution..

I had to use the second version of ‘add_rewrite_rule’:

add_rewrite_rule( 
  '^products/directory/?([^/]*)/?',
  'index.php?post_type=acme_product&c=$matches[1]',
  'top'
);

I needed to register a query var,, since external query params aren’t recognizable to WordPress

function 123wp_query_vars( $query_vars ){
  $query_vars[] = 'c';
  return $query_vars;
}
add_filter( 'query_vars', '123wp_query_vars' );

Then retrieve the value in the template using:

get_query_var( 'c' );