I had this same issue. I think I had Progressive back in the day when I had my 1967 Series IIA. Basically I would get it appraised once a year (it was around $100) to have an "expert" say that it was worth x, based on the unique market for that vehicle, and then it was insured for exactly x.
It's actually a good thing in a weird sort of way, as this way if something were to happen to the vehicle there should be no quibbling over how much it's worth, they should pay you exactly x. However, generally this means the insurance costs more.
With any other car, if there is a complete loss, at the time of the loss, they look at vehicles of similar condition, make, model, in the area where you live and cash you out on an average. Because Series vehicles are more rare, if there wasn't a pre-agreed value it would be a bitch for them to come up with a figure.
I also believe the "gray market value" term was thrown around allot too when I was looking for insurance for it - as they consider it to be one of these vehicles if it was uncommon.