Jerry makes a point that i was going to make, so i'll just try to state it more clearly. Someone 15 years old in 1969 would be 52 years old today. That's old enough to have wads of cash, in their prime earnings years, well into their mid-life crisis, etc.
You won't find many 1985 bikes selling for $3600 because most of the buyers are too young to be nostalgic and too poor (age 36) to have the deep pockets for this.
The 20" size (sorry for the typo) might make it a rarity. Imho most bikes are 22", and when in doubt, i think the smaller bikes sell for more than the larger bikes.
- Don Gillies
San Diego, CA