BTH British Thomson Houston - Duplex Speakers

Here is some vintage British BTH duplex speaker porn. The BTH speakers are pretty rare and I have a hard time coming up with documentation. These are examples of two of their duplex speakers. I believe the cabinet is made by Lockwood which I am unfamiliar with but will start researching. If anyone has scans of BTH documentation please pass them my way. The theater sound systems they did were very nice.

Other than keeping foreign materials out of the driver does the covering serve any other function?

This driver is beautiful. Not sure how it sounds but I really like the looks of it.