The Spendor BC1 and basic follow-on sibling LS3/6 from 1973 or so, used the 'super tweeter' to allow engineers to better hear line whistle and so on.
Interesting that the BBC bought far more BC1's - a few hundred pairs I gather and only a tiny number of their official version, which was similar but with the Celestion HF2000 'super tweeter' and a level matching auto-transformer on the input, which further increased the bass distortion and lost even more amp control (used domestically, vinyl sources of the time plus more powerful amps were a death-threat to the frail original bass drivers which were very easily 'popped').
P.S. The BC1 (and LS5/5 as I've read it) DID NOT have the so-called BBC Dip, the BC1 actually measuring quite flat in the lower kHz region. My BC2's with larger diameter voice coil (no pole extension on this basic driver, used in the SA2 and as mid driver in the BC3) do go a bit awry at 1.5 to 2 kHz due to phase issues without the longer pole extension added in the subsequent SP1 driver derivative. HiFi Choice measured many BC1 samples during the 70's and 80's and the main difference as time went on was improvements in power handling and low bass distortion
View attachment 279805
The 4kHz dip (quite bad in the top plot on tweeter axis) disappears five degrees above axis as you can see and this is perfect when using the low trolley stands that most were sold with. I should add that in the late 60's and early 70's, the BBC did much work on dispersion patterns, but not sure how much of this is relevant today as things have moved on so much with CAD to help and so on.
Sincere apologies for this thread drift. I suspect all thoughts on the Dave are now exhausted.