This is the Multi shot breech for Crosman 1377 OR 1740 pistols made by GMAC in UK.
This kit can also be used on a 2300T (.177 cal)
Kit includes everything shown in the pictures and including one magazine(no air guns or scopes or sights are included).
Stock barrel doesn't require any modifications to install this kit. Stock breech screw will work on this breech hence a new screw is not included.
You will need new Barrel band though as the breech is a bit taller than the stock breech. Please see Related products section to select the appropriate barrel band