How to add a (second) custom sku field to WooCommerce products?

Adding extra fields can be done in a very simple way:

function jk_add_custom_sku() {    $args_1 = array(        'label' => __( 'Custom SKU', 'woocommerce' ),        'placeholder' => __( 'Enter custom SKU here', 'woocommerce' ),        'id' => 'jk_sku',        'desc_tip' => true,        'description' => __( 'This SKU is for internal use only.', 'woocommerce' ),    );    woocommerce_wp_text_input( $args_1 );    // Extra field    $args_2 = array(        'label' => __( 'Nominal codes', 'woocommerce' ),        'placeholder' => __( 'Enter nominal codes here', 'woocommerce' ),        'id' => '_nominal_codes',        'desc_tip' => true,        'description' => __( 'This is for nominal codes.', 'woocommerce' ),    );    woocommerce_wp_text_input( $args_2 );}add_action( 'woocommerce_product_options_sku', 'jk_add_custom_sku' );// Savefunction jk_save_custom_meta( $product ){    if( isset($_POST['jk_sku']) ) {        $product->update_meta_data( 'jk_sku', sanitize_text_field( $_POST['jk_sku'] ) );    }    // Extra field    if( isset($_POST['_nominal_codes']) ) {        $product->update_meta_data( '_nominal_codes', sanitize_text_field( $_POST['_nominal_codes'] ) );    }}add_action( 'woocommerce_admin_process_product_object', 'jk_save_custom_meta', 10, 1 );