Great question!, I never see them come up eather. My guess is there under beds, in closets and attics. The older one's (75-80) The production #s will be very small, so that ads to rarity.
I dont think they have fallen apart Bob and Kurt built them to last with there bolt on necks.
I'd love to aquire a 74-76 Fat chance the oldest one i've seen for sale recently was a 79.