For post meta

// WP_Query arguments
$args = array (
	'p'                      => '771',
	'meta_query'             => array(
		array(
			'key'       => '_filled',
			'value'     => '1',
			'compare'   => '=',
			'type'      => 'NUMERIC',
		),
	),
);

// The Query
$query = new WP_Query( $args );

// The Loop
if ( $query->have_posts() ) {
	while ( $query->have_posts() ) {
		$query->the_post();
		// do something
	}
} else {
	// no posts found
}

// Restore original Post Data
wp_reset_postdata();