It seems that if the membrane has one physical connection to the PCB it is a newer version. If it has two physical connection it is the older dual membrane units. I have not been able to find a replacement dual membrane.
Yes, what he said^. When you see the KB with the case off, the Mitsumi boards will have a single ribbon flex cable going to the small PCB in the upper right corner. The PCB will only have one flat flex cable receptacle.
Without taking the keyboard apart, I can only see a single ribbon between the keys and the PCB. The motherboard is Rev6A so I hope that bodes well for having the newer keyboard membrane. I did see the word Mitsumi printed on the PCB.
You're good to go then. I ordered a replacement membrane from the UK a few months back and it works flawlessly. Very good condition stock they have at Vesalia's listing (can't find the link atm).
Good to know. I currently have a refurbished floppy drive in transit. As soon as the A500 gets into Workbench, I can check the keyboard. Hopefully I can start using the computer and just order a membrane to keep handy.