See if you can find anything from the Yahoo Spendor group.
There were slightly different versions of the HF1300/Coles setup made, mainly impedance differences, but this may have an effect on sensitivity. The Spendor versions were higher impedance ones with larger rear magnets IIRC. The standard version wasn't quite the same magnet-wise as I recall.
The high efficiency Celestion Ditton 25 used twin HF1300's to get the treble sensitivity up. The catastrophic effect on dispersion was not a joy to hear, the treble sounding smeared in the Ditton 25 implementation - not an issue back then, but Ditton 44's were much better and the 66's were shedloads better
The HF1300 has a very narrow bandwidth by modern standards, nasty resonance at 3.5KHz which needs a steep roll-in at 4KHz or so and a peak at 14KHz followed by a nosedive into oblivion, hence the Coles, coming in out of phase at 10 - 12KHz to smooth the 14KHz region and extending the EHF smoothly thereafter. Seems a right bodge but it worked well then and still sounds clear now IMO.
Don't despair of modern tweeters though. The ring-radiators made by Scan/Peerless? these days are very good if extremely expensive. Check Falcon Acoustics out on their re-vamped website. These modern tweeters are scrupulously clean, clear as a bell and have wide bandwidth, better for simple crossovers.