The aquaplast looks consistent with a factory job, which was never done for production horns to my knowledge. I would guess they started life as 511E's, and had the throats exchanged to the small format. Can't say that I have seen this before. Original 511E's have become few, but if someone had them and wanted to use small drivers, and dampen ring, it may be easier tha applying aquaplast effectively to 511B's.
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