Theo

I have a hunch that an old manual typewriter might give the right appearance.

That would make sense Dave. Italian reed makers supplying "tuned" reeds usually supply them a few cents sharp because doing the final in situ tuning is slightly easier tuning down than up.

The catalogue page on their website illustrates the current Hayden Duet, scroll down to find it. Does this show an improved layout?

Jeffries ends are definitely not tin! The nickel silver material is often finished with nickel plating which can show as a thin surface layer when it wears through. Hard soldering won’t damage the metal but the soldered area will need to be polished afterwards. Jewellers understand this.

Apologies, my mistake.

No, it is 11 steps. There are 11 intervals between 12 notes.

Of course it might not be the reed that is losing air. Their might be an air leak from the reed chamber by another route. Where that could be depends on the construction of the concertina, so you would need to first answer the question above from Stephen Chambers before we could suggest where to look. .

Bellows don't need any maintenance apart from brushing off dust and keeping the the whole instrument away from extremes of heat and cold and from dampness and extreme dryness. Do not apply leather dressings, they can do more harm than good.

If you can play jigs and reels for dancers, and the dancers are smiling, then you have found a good pace. Difficult these days I know when we can't get together.

I have a guitar playing friend who has an octave pedal that plays the guitar an octave lower than sounded. I see no reason why it wouldn't work with a concertina, but you would of course require a pickup, amp and speaker. Some examples here

I've come across this problem a number of times where the cranked lever wears unevenly as described by Geoff. The actual wear takes place in a tiny area on the side of the lever where it passes through the support post, and on the inner surface of the window in the support post. It's often almost impossible to see the wear unless you remove the lever and the post and examine them through a lens. You can usually see the result of the wear because the lever will appear tilted over to one side. Comparison with nearby straight levers is helpful here. When one replaces a Lachenal support post

Probably not much help. On a GD concertina the D row is higher in pitch than the G row, on a DG button accordion the D row is lower in pitch. Consequently any cross row fingering patterns don't copy across from one to the other.

Leather accordion valves can have similar variability to that described above by David Elliott for concertina valves. I don’t think the Mylar reinforced leather valves have been around for long to judge their stability over several decades.

To get a better gradation of stiffness accordion valves use multi layer construction with 2 or 3 layers. The upper layers are be made shorter and they can be shortened further to make fine adjustments to the stiffness. They do have another disadvantage of being more noisy, especially in larger sizes. It is normal practice now among some high quality accordion manufacturers to use plastic valves for the smaller sizes and leather for the larger, or even leather with an added layer or two of plastic.

I have some old German reed plates. Don't know what key, but you can have them for postage cost. I also have some reed plates from old German melodeons. They are twice as long of course, but I can't see whey they could not be cut in half.