Doug,
thank you again for your interest.
I have finally found the origin of the device through the net: the name is
Trident 300 and made by Spencer Carter, Cornwall UK. Useless to say that
the manufacturer does not make them anymore but passed the task of customer
support to a nearby company (www.staytensioner.com) where Les, a nice guy,
helped me. Now that I have new seals and a drawing and will soon fix the
thing.
How did I finally find all this? Through another very nice guy at
Nautors-Swan who knows the wherabouts of (almost) all the swans in the
world!

Well, thank you again and keep in touch
Daniel