Evening Peter, this is an SKN marked example, (SKN 721) I guess an armourer back in the day must have fitted and slotted a full length barrel and stocked her up with the viewing cut outs for training purposes.

The barrel,(1948 Faz) in this case has a 5 inch slot on the left hand side, starting in the last inch the chamber. It's been plugged with weld forward of the cut channel.