We know the c12 was produced for a year, so I am assuming that BSA didn't really push the boat out in sourcing parts unique to it, I'd say more of a bitsa from what they had left in the factory.
So, I'll pose a question.
What other models used the same seat as the c12?
There are lots of similar seats, but the c12's seat had two grooves in the bottom of the pan. This was to allow it to sit closer to the frame.
Others have these grooves standing proud, which makes the seat stand higher than it should and makes it look a little ungainly.
Hopefully I have explained that in a way that people understand and hopefully can help me find a dual seat to fit my c12
I think it was unique to the C12.
The best seats, not cheap ones, come from R K Leighton, worth phoning them to enquire.
http://www.rk-leighton.co.uk/