Nice! Looks like you're covering a lot of the possibilities. Other thoughts I had:
* Can any signal can be detected? (like placing near a radio makes the sound go all distorted)
* Are they all perfectly balanced? Or perhaps lopsided in some way, where one side is much heavier than the other?
* How heavy are they in general? Does that seem reasonable based on the density and type of wood involved? (if not, perhaps something is inside, etc)
* Perhaps gentle chemical tests can be used to spot a number of things, such as the presence or lack of wood glue (no idea if this is possible, just spit balling)
Man I could go on forever just thinking of possibilities haha. I am just still floored that you have 9 of them together in your possession.
Feel free to edit your post if you think of anything else, I am always eager to take on new ideas. Until Coin 10 becomes known to be in existence it is up to our imaginations as to how we proceed.
I 99.9% will not try anything that damages the wood, even though it could be a possibility I would rather have these presented at a museum or a potential sale to someone that has the treasure hunting bug.
I too am floored that you have 9 all together, how long did that search take place?
My first two coins came through the door on October 24, 2017 and the last(ninth) came February 11, 2019 for a total of 475 days start to finish.
I 100% thought it was impossible and that I would end up with at most 6 or 7 of them. Ran into some obstacles along the way at each step.. only the first two coins were ever for sale in an auction. Everybody else I tracked down and had to make appropriate offers that would entice each owner to want to give up their coin. All it would take was for me to rub one of them the wrong way and my collection would have been incomplete no matter how hard I tried. With each owner I was completely transparent about how many of the coins I owned at the time and that it was my end game goal to reach all 9.. I knew this could potentially inflate the amount that they would want for their coin but I never wanted to pull the wool over anyone's eyes.
The journey taught me a lot of patience and if there is a Coin 10 out there I am very capable of waiting much longer until the right set of eyes end up on this thread.