^ I know nothing of tandems but the lugs on yours look typical of 1930's/1940's frames.
The headset is a Brampton and spares for this are easily obtained as George Longstaff machined a big batch which are now stashed away at the Tandem Club - same goes for bottom bracket sleeves, brake linings and eccentric BB shells and all the other specialist stuff - the tandem club shop can be found here - http://www.tandem-club.org.uk/_clubshop.htm
^ I know nothing of tandems but the lugs on yours look typical of 1930's/1940's frames.
The headset is a Brampton and spares for this are easily obtained as George Longstaff machined a big batch which are now stashed away at the Tandem Club - same goes for bottom bracket sleeves, brake linings and eccentric BB shells and all the other specialist stuff - the tandem club shop can be found here - http://www.tandem-club.org.uk/_clubshop.htm
If you're keen on finding out what it is you've got, your best bet is probably the tandem club forum http://www.tandem-club.org.uk/_dboard.htm
Hope this helps