Malcolm posted a link to Sugawa's web site which shows a matchlock that has been converted to a breech loader, from what I understand from the translation of the original Japanese it is called "KoSo-shiki". You can tell it is a Choshu from how far the barrel sticks out from the front of the stock, 130mm long.
11-4, breech-loading weapon of Kazuju Gun to remodeling, was further modified to Arubini method Choshu of matchlock the (without a signature) to the tube out. Closure fixed by spring out bolt to the rear from the breech, release use the front of the handle of the hammer. The original gun was having a high rating.