THere were THREE sorts of anti tank rifle. The Mk1, the Mk1* and 2............. Sorry, you mean two types of muzzle brakes. The first one kicked uip so much dust and shxxe that it was akin to standing up while waving a big flag at the other tanks and shouting '.......cooeeeeee, we're over here'. So the Harmonica type was introduced and the round one declared obsolete. The Harmonica one was designed by the Bell Punch Machine company.

It makes you wonder about the men who designed some of this stuff. I mean........ who in their right mind would think of using a zipper in the desert, The modern nylon ones are marginal so the big old original brass things must have been useless! Maybe that's why it was changed to the round type with a flap