Hi both
Partial success only I'm afraid. It turns out that this is a beefier unit than the one one my SuperSeal, and the spring is held captive in the upper unit by a plastic collar. You'll forgive me for not drilling out the rivets to measure the spring, but I have instead measured the length of the whole unit in its 'resting' position i.e. disconnected with no weight on it, and it is 132cm from pin centre to pin centre. The distance from the top pin when connected on the boom to the back of the mast is 100.5cm. I hope this helps.
It did however occur to me that you might want to use these measurements just as a guide: presumably a different combination of sail/battens/blocks etc will weigh differently, and require a different length of spring to get the same result? I removed mainsheet blocks that would have safely warped the Titanic off an iceberg, so I reckon my boom now weighs a lot less than it did, as well as looking less like a Thames Barge. I would recommend cutting carefully and reducing length gradually until you are happy with resting height.
Good luck!
Peter
Peter Dann
Blue Moon 325/32