It's a big hunk of aluminum there. And such a large chunk of aluminum is probably prohibitively expensive these days, not to mention shipping. Plus, we'd expect one would need a foundry to cast them... there seem to be modern knockoffs coming out of asia.
This gets more into horn theory than I am blessed to understand, but suffice it to say they are relatively different horn types. (e.g. between 811/511 sectorals, and MR's)
Other than whatever cutoff frequency the desired MR is, the other difference being that MR are CD (constant directivity) horns, and the sectorals are not.
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