Archive dropdown for custom post type

I am tring to figure out how to have a dropdow that shows an archive of posts from a custom post type. I am using the code below but I cant figure out how to only show posts from the letters post type.

Thank you for any help on this.

<select name="archive-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;">
  <option value=""><?php echo esc_attr( __( 'Select Month' ) ); ?></option>
  <?php wp_get_archives( array( 'type' => 'monthly', 'format' => 'option', 'show_post_count' => 1 ) ); ?>
</select>