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Nothing in particular except to show another experiment, a diaphragm made of plastified aluminium foil from a wine bottle cap, grooved by hand using this tool, just ready for testing.
Finger scratching sounds much like the Meltrope soundbox.
The soundbox is my experimental one, which has the balance springs relocated in a different way, so they press on top of the needlebar fulcrum plate by means of inserted steel balls, so the compliance of the needlebar is larger. Similar to the balance springs of the EMG soundboxes... but using steel balls instead of knife points. The system is like the two spring EMGs, they press the plate against the knife points and at the same time can provide extra spring action to counterbalance the needlebar movements, depending on the position of the balls with respect to the knife points below the plate, the reaction can be adjusted easily. Supposedly, this improves the needlebar response, making the return faster. They also help damping. The fact is that the Exhibition springs placed as usual do a similar effect... I've also inserted felt washers between the springs and their screws, to make all it more compliant.
What is true is that the needlebar vibration is more free with this system, as the original springs arrangement makes them exert a very strong counterbalance action.
Done photos to illustrate all this...
It's just another test. Those colleagues who have seen other experiments are already familiar with my perseverant but non satisfactory experimental Exhibition.
I'm always using my modified rubber back with tracking angle correction, just half of a 135 degree copper elbow attached to the back so it is inserted on the tonearm and the resulting angle is reduced, the needle runs almost parallel to the groove tangent and the teaching angle is greatly improved.
The photos show the tools used to make the diaphragm and to install it, the soundbox with the special elbow, and the fulcrum arrangement with the balls, relocated springs and green felt washers.
I like the black diaphragm, it reminds me of the first version of the Viva Tonal soundbox...

Did you make a sound comparison with other soundboxes? How does it rate against them?

Yes, Carlos... subjective comparisons only. Compared to the standard Exhibition, it has more bass, but the sensation is that there is less treble. It can be the balance altered by the bass presence, of it can be a guilty diaphragm. The material I'm using gives to me the sensation of not transmitting properly the high end treble.
It could also be that the soundbox body is too small for the power of this diaphragm.
Of course, the comparison with the 5a is nonsense. It's very difficult to match the performance of that soundbox. It is even better than the Viva Tonal no15, maybe because its design is better (the spider, the back shape replicating that of the diaphragm, the bullet or phase plug... These refinements are, no doubt, who make the 5a and 5b so pleasantly balanced, with bad and high treble equally present in the sound.
I need to test pure aluminium foil, with no plastic reinforcement, and also try to include a spider. I'm convinced that this will improve the treble.
The actual sound of these experiments is a powerful sound, no doubt, but lacking treble and sounding kind of 'tubby', as when one shouts with hands cupping around the mouth...
If I can have a quiet while at home alone tomorrow, I'll record a video with the results with electrical and acoustical 78s to compare.

Iñigo, I agree with you that trying a pure aluminium instead of a plasticized diaphragm will probably be better. The Expert and EMG soundboxes, that to me are the best offsprings of the Exhibition, both utilize aluminium with great results over the spectrum. Your experiments are quite interesting!

It's kind of an eternal bug... I cannot stop trying things. This beautiful looking Black Diaphragm Exhibition neither sounded well, worst than the original one. Next trial will be with aluminium, or I'll install definitely a fine glass diaphragm and let it be forever...
Besides that, the gramophone I'm using for all this is the HMV Style III hornless tabletop, later model no58 in the UK, similar to the VV-VI, which can bear a HMV no5a soundbox with great results... At times I'm using this gramophone as a daily player in that configuration, and the soundbox extracts the best of all records, despite the obvious limitations of its small horn.

I wonder what a thin wood veneer diaphragm would sound like, sealed with shellac.

I must have got this diaphragm in a lot box of parts I bought some years ago.
It looks like wood, but I don’t think it is.
One can see threads at the edge, so maybe it is a coated fabric that was pressed in a wooden mold.
I have not tried it as yet, but am keeping an eye out for a suitably sized reproducer to mount it in.
Diameter about 2 5/32” or 55mm.
Thickness is .0065” to .0068”, about .173mm, depending on where on the edge I measured.
The photos are back-lit, as the diaphragm is semi-translucent.
Flexing it in my fingers, it is firm in the centre rings, while the outer edge feels more flexible. A promise of good bass response?
Anyone have an idea of what it came from?

What a strange thing... Reminds me of the paper-fabric material of certain horns from 1940s American portables, Birch and others

Ha! I just measured a Graphonola diaphragm and it is the same size. I will try it in there soon and report.

Inigo,
I particularly like your experimental mindset, it allows you to look at things differently and search for improvements. I think that diaphragms made from unusable wooden instruments, like violins, cellos, etc. might give a good result since they are made from wood that resonates sound or similar wood veneer. I think these were made back in the phonograph era.