The Bren light machine gun is far from a forgotten weapons, but some of the accessory bits that were used with it are pretty rare today. The 100-round drum magazine issued for anti-aircraft use are one such item.
You might be wondering how you are supposed to use the sights with that drum in place – well, you don’t. The drums were intended for antiaircraft use, and thus the guns using them would have taller AA spider sights installed. Some were also uned on armored vehicles, but this was more of an afterthought and not the original intent. The drum requires an additional mounting bracket to hold the front end in place, while the rear of the drum has a set of feed lips like a regular box magazine and snaps into the magazine catch. As you can see below, the drum adapter does not preclude the use of standard box magazines.
We have a few more photos of one of these drum magazines and its setup on the gun (click here to download them all at high resolution):
Edit to add: Oleg Volk has a great photos of one of these drums as well, including one of the piece disassembled: