Yeah - took a long time to source all the period correct parts. Our tandem is also quite rare - it's a Bicycle Built for Two as opposed to the Twinn which you have - the difference being the top tube for the captain. It was only made for two years and came in blue or red.

I'd just sell it for what someone is willing to pay - unless you want to do what I did and I wouldn't recommend that to anyone who is sane