Mica diaphragm w/ metal rim

[B.B.B]

A few months back I got a bunch of NOS diaphragms of off eBay, mostly "Quaker" but some were in a plain envelope, no brand
One was a conventional mica, fitting to a No4 soundbox, but with a metal rim around it.
The rim was clearly factory-mounted.

Any clues, common knowledge? I haven't seen one before. Any ideas of the use?

metal_rim_mica_no4.JPG (24.05 KiB) Viewed 797 times