A pair of mint condition LE20-1 was bought on eBay. These are the early alnico versions of cone tweeters from James. B. Lancing. "Much sought after!" as the saying usually goes on eBay. Well, we'll see if the LE20 tweeters can live up to this.. Beautifully made they are with the die cast frame holding the diaphragm. Removing the frame exposes the voice coil and three small felt pads glued to the frame and diaphragm, possibly to control resonances or help keeping the voice coil centered in the magnet gap. Some corrosion to the pole pieces, but nothing that disturbs the voice coil from moving freely, so I left it there.

Left: SPL measured at 0.25 m distance and normalised for 1 meter/2.8V. The LE20-1 has a minor peak at 10 kHz and declines rapidly above this frequency.
Right: Impedance of the two tweeters. Fs = ~1600 Hz due to rather rigid paper suspension.

Left: LE20-1 sample 1 distortion test at a level equal to a 90 dB sound level at 1 meter distance. Distortion level is rised by 30 dB.
Right: Sample 2, same conditions as sample 1.
Compared to LE25 and LE26 these older versions display significant levels of 2nd harmonic distortion.

Left: LE20-1 sample 1, cumulative spectral decay displaying decent performance at ~2-9 kHz. The peak at ~10 kHz is noticeable, but seen worse.
Right: LE20-1 sample 2, same condition as sample 1.
If used above 3 kHz, these tweeters have a fairly clean decay.

The LE20 tweeters may need a supertweeter by modern standards due to limited response in upper octave. Sensitivity is quite high around 95 dB/2.8 V. Generally I would pick a LE26 tweeter over the LE20, providing extended response in upper and lower octaves. I haven't tried setting up the LE20 in any 2- or 3-way systems but I would expect them to be useful from 3 kHz despite high 2nd harmonic distortion.

Left: SPL from samples 1 and 2 normalised for 1 meter/2.8 V input.
Right: Impedance of sample 1 and 2. The LE25s have softer suspensions compared to the LE26 and possibly slightly lower moving mass as the Fs appear to be the same as LE26.

Left: Sample 1 with foam on faceplate (red) and with inverted LE26 faceplate (blue).
Right: Sample 2 with foam on faceplate (blue) and with inverted LE26 faceplate (red).
Sorry for swapping colors.
The LE25 appear to be sligtly more sensitive compared to the LE26. The lower impedance (4 ohms) may account for the increased sensitivity.
The LE25s do well up to 16-17 kHz but are quite peaky at 13 and 16 kHz, which shows up on CSD as well.

Left: Cumulative spectral decay of sample 1.
Right: Cumulative spectral decay of sample 2.
I've had the L26-3-ways running for a considerable length of time and do not find the 13 kHz peak noticeable. Peaks always look awful on CSD graphs and they may be 5 dB above average level but appear to die out fast.