Derek,
The Lahti mag was designed to feed up. It is double stack and ends up as a single feed in the center of the mag. The Bren is double stack and grabs a round from either side. The main portion of the mag is much wider than the Bren well. The one I have will not fit the current Russian7.62 x54r. It is not long enough. It was designed for a 7.62x 53R. The
rounds are for all practical purposes identical. It can be modified to fit the 7.62x54R by removing a curved plate in the mag. There is a very long Thread over on the Weapons Guild about adapting it to other rifles, nothing about the Bren. Just about everyone had very little luck.
I'll send you one. Feel free to experiment with it. I don't need it back.
Joe