This makes 4-5 survivors that I am aware of - one that I own, one I am trying to buy, one in Wisconsin, and I seem to remember seeing photos of one in Europe.
And this is with my somewhat agressive looking for them. So, yep, they are rare - but they don't seem to be valuable. The one I own now is missing a lot of parts, and has a lot of rust damage - yet I sat out to restore it.
The revolving platform is to do telephone line repair work from. Sort of like today's bucket truck, but simpler. Pull up next to the pole, swing the platform out and suddenly you are standing next to it.
Hope this helps!
Sincerely,
David Doyle