It's a 15" 3859 dual-concentric which, in the mid-1980s, was originally sold in either the Panther or Cougar stage monitor cabinet. A variation was briefly (and unsuccessfully) marketed for home audio and my partner has come up with his own design, loosely based on the Monitor Gold x-overs.
The actual value in question can vary from 1.5uF to 2.2uF. It's run in parallel with a resistor and I believe that by varying the value of the resistor we can get the cap down to 1.5uF. While there is a resemblance in principle, it differs somewhat from the published Tannoy x-overs of that period.
Ultimately, we'll be applying what we learn to the Monitor Golds, as well.
Joe