It has been years since I had this pistol. I sold my auto pistol collection about 1972. I don’t recall what the projection at the back top of the magazine is for. If I saw a good photo (like the ones already posted - excellent!) of the back of the magazine, it might help me to remember. If it is part of the follower and sliding in a groove on the back of the magazine, it probably operates a hold-open device. I don’t really recall if the Brixia had such a device or not. In a lot of Italian pistols, it is simply the square back of the follower that holds the bolt or slide open on the last shot, with the slide going forward when the magazine is removed. If the projection is part of the back of the magazine body, then I don’t have any idea what it is for.
This is a rare magazine for a very scarce pistol. I have no idea what the value is, but probably in excess of the value of some new inexpensive handguns. Certainly US$100.00 or more. I am not into values, either for gun items or cartridges, as they simply are not a consideration to me in my collecting. I buy what I can afford or what seems to be fairly priced, and I don’t collect based on potential value at all, but rather just for my own enjoyment and knowledge. That leaves me a poor person to rely on for values. As a retired professional dealer, the knowledge of pricing I had then is now about seven years old - an eternity in pricing collectible gun items, judging by the almost insane prices I see in advertisements.