(Identify not Inner Diameter) :-)
Does anyone know the model of the crank? It doesn't seem to have any identifying labels, but I imagine the selection for tandems is prety limited.
Thanks.

Should be some numbers stamped on it somewhere. Sometimes they mean something. If it is really carbon, might not be stampable. Maybe look on ebay for FSA cranks and see if you can match by eyeball method. I have seen CF tandem cranksets on there several times but didn't pay attention as I don't do anything CF except for golf clubs and I suck at golf despite awesome clubs...

Yeah, I tried that first and failed; but now I've tried it again and came up with this interesting info...
https://www.ebay.com/itm/New-FSA-Carb...item43b95a72d6

Why not call FSA, they are the experts, I have talked to,them a couple of times and they are very helpful.

Looks like it's the old Carbon Pro Elite